A good index enables the user to navigate sensibly through the work’s main topics and facts. How long it needs to be to accomplish this depends on the size and complexity of the work and the requirements and expectations of the readership. In general, a short index for a general book can account for as little as 1 per cent of the text it catalogues, while an exhaustive index for a specialist book can take up as much as 15 per cent. The editor should be consulted before preparing the index to a printed book, as length constraints may apply, such as those needed to make an even working (see 1.8). In digital products, there are no such constraints, and a professional indexer will achieve a better result if allowed to determine the index length.
The index is a vital component of the work, directly affecting the text’s usefulness for the reader. This chapter provides some general guidance on producing and checking an index for those not trained in indexing. However, if you are an author who needs an index, especially for a large or important work, it is strongly recommended that you consider using a professional indexer. Not only are they skilled at choosing, compiling, and ordering an index’s content using specialized software but they also bring a fresh pair of eyes to the text, which is invaluable in assessing the content from the reader’s viewpoint. Indexing software also reduces the problems associated with changes in pagination and ensures typographical consistency. Indexes are best submitted electronically, as rekeying errors are hard to spot.
Although at first glance free-text searching in digital products might seem to make indexes redundant, the chief disadvantages are that users must know precisely what they are looking for, and how to spell it; that multiple hits may be generated with no indication of which are of greatest relevance; and that underlying concepts which do not have obvious key terms are not retrieved. A well-formed index is therefore just as important in digital as in print publications.
Indexes are often created from first or second proofs, i.e. late in the production process, using page numbers as locators. With the exception of ebooks where fixed layout is desirable (such as highly illustrated titles), the possibilities offered by reflowing the same text into a great variety of digital formats means that the page is irrelevant as a concept other than for the print edition. This brings a major advantage to the indexing process as it can now start at a much earlier stage, as soon as a stabilized text becomes available. Two methods of indexing unpaged content are described here: embedded indexing and indexing to location markers.
Embedded indexing (EI) involves inserting actual index terms as hidden text marked with a functional identifier at appropriate locations in the electronic version of the indexed document. After pagination, EI software builds the index automatically by associating the final page locations with each embedded index entry and performing the sorting, suppression of duplication, and formatting necessary to produce a usable index. Most of the common word-processing packages, including LaTex, include an EI system. The default is usually to produce an index with page locations, but an embedded index can be presented with hyperlinks for use in digital product. Because of its availability in word-processing software, embedded indexing is preferred by some publishers and by many authors producing their own indexes. It may be more time consuming for a professional indexer than production of a conventional index using specialized software. Authors making their own embedded indexes should also be aware that, unlike specialized indexing software, the default alphabetization routines of the word-processing packages may not fully implement the principles outlined below under 19.3 (though some systems include mechanisms that can ‘force’ the correct order). Authors planning to submit an embedded index should consult their publisher early on in the production process.
An alternative approach is to mark the point in the text to which an index term is to be linked using a location marker rather than a page number. The location marker (known also as an ID, unique number, or tag) must be unique to each point to be marked. Apart from the use of location codes rather than page numbers, the construction of the index follows exactly the same course as a traditional index. It differs from embedded indexing in that only the location codes, not index terms, are inserted in the electronic text and the indexer compiles a separate, stand-alone index file.
The indexer’s job is to identify and analyse concepts treated in the text so as to produce a series of headings based on its terminology; to indicate relationships between concepts; to group together information that is scattered in the text; and to synthesize main headings and subheadings into entries. All items of significance (names, places, concepts) should be entered, with correct page numbers and spelling. The needs of the user of the index should always be kept in mind, particularly in terms of what and where things will be sought: for example, in all but the most technical books an entry for humankind or mankind will be more helpful than one for homo sapiens. Looking at indexes of related works in the field can provide useful guidance.
Usually a single index will suffice: multiple indexes can be unhelpful and should not be provided without being agreed with the editor beforehand.
Indexes comprise a list of single headings or multiword headings followed by one or more locators (a page or page range, section, numbered paragraph, or some other division) to indicate where the topic is discussed in the text, or by a cross-reference to another heading. Entries complicated enough to require further division may have subentries within the main entry. In all but the most complex indexes, subentries within subentries (sub-subentries) should be avoided.
In some works it is desirable to highlight those references that include the principal discussion of a heading, and this is usually indicated by the use of bold type:
miners 245–7, 257, 346
Editors of multi-author works must ensure as far as possible that contributors’ terminology and sources have been standardized to a single form throughout a work: the index may otherwise require frequent cross-references to guide the reader between variants.
Main entries are those most likely to be first sought by the reader, and should be in a form that anticipates where the reader will look for them. They should be concise and consist of nouns modified if necessary by adjectives, verbs, or other nouns; unless house style dictates otherwise, they should start with a capital letter only if the word is capitalized in the text. Normally choose the plural form unless the noun is uncountable, e.g. lightning. If use of the singular is unavoidable, both can be accommodated through parentheses: cake(s). If singular and plural forms have different meanings, both forms may be used in the index.
Ignore passing or minor references that give no information about the topic. Do not include entries from the preface, contents, introduction, and other preliminary matter unless they contain information not found elsewhere that is relevant to the subject of the work. There is no need to index bibliographies or reference lists. There is usually no need to augment a heading with supplementary information from the text, though in some cases a gloss or other clarification in parentheses may prove necessary, such as in vom (proper name prefix).
Use cross-references or subentries where a single reference spans ten or more pages, or where lengthy strings of locators threaten to clutter the layout. An array of unqualified or undifferentiated locators several lines deep is tiresome and unhelpful to users, who will have to spend too much time trying to locate the information they seek. Any string should ideally be reduced to six or fewer numbers. For example,
habitat loss 83–5, 100–7, 114–16, 117–18, 121–2, 125–9
can be broken down into:
habitat loss:
from development 83–5, 100–7
from erosion 125–9
from logging 114–16
in Asia 117–18
in England 121–2
It is advisable to avoid main headings that echo the title or subtitle of the book. If such entries are essential, they must be succinct. In biographies or collections of letters, keep subentries relating to the subject to a reasonable minimum, confining them to factors of relevance.
Subentries are used chiefly to analyse a complex subject heading made up of two or more discrete categories:
life:
beginnings of 2, 98
DNA’s role in 5–7, 10, 12–13
and inorganic matter 7, 10, 28–9, 48
as process, not substance 10, 11
understanding 240–1
Run together a simple heading with no general page references and only one category:
life, beginnings of 2, 98
Sub-subentries can most effectively be bypassed by denesting the subentry containing the sub-subentry into a separate main heading of its own, cross-referring to it as necessary. For example, in the following the subheading industries under moorlands can be changed to a cross-reference ‘see also industries’, leading to a main entry with subentries of its own.
moorlands:
enclosure 198, 200, 201
industries 201, 205
charcoal-burning 197
coalmining 201
tin-mining 197
roads 201
It may not always be possible, or practical, to use subentries and sub-subentries to avoid long strings of locators in an exhaustive index—such as one containing numerous references to authors of cited publications, a separate index of authors, or an index of musical works.
An index is not intended to be an outline of the entire text: there should not be a subheading for every locator, and a list of subheadings all with the same locator should be condensed.
Notes should be indexed only if they give information not found elsewhere in the text. When there is a reference to a topic and a footnote to that topic on the same page, it is usually sufficient to index the text reference only. See 19.6.
Cross-references are used to deal with such things as synonyms, near-synonyms, pseudonyms, abbreviations, variant or historical spellings, and closely related topics; they fall into two classes. The first, introduced by see, directs attention from one possible entry to a synonymous or analogous one, under which the references will be found:
Canton, see Guangzhou
farming, see agriculture
Dodgson, C. L., see Carroll, Lewis
Severus, Sextus Julius, see Julius Severus
The second, introduced by see also, extends the search by directing attention to one or more closely related entries or subentries. Two or more cross-references are given in alphabetical order, separated by semicolons:
birds 21, 88–9; see also chickens
clothing 27, 44–6, 105–6; see also costume; millinery
housing 134–9, 152; see also shelter, varieties of
tread depth 109; see also routine maintenance; tyre
condition, indicators
Except in the case of reference from a non-preferred to a preferred term, or cross-references between acronyms and their full forms (see later in this section), do not use a ‘see’ cross-reference to a heading that takes up the same space as that occupied by the cross-reference itself. In
authors, see writers
writers 25, 36–8
the reader would find it easier if the locators were repeated after both headings (a ‘double entry’). Equally, do not cross-refer simply to the same references listed under a different heading, nor bewilder the reader by circular or redundant cross-referring:
authors 25, 36–8, 50; see also writers
writers 25, 36–8, 50; see also authors
There must be no ‘blind’ cross-references: in other words, ensure that every cross-reference is to an existing entry. Cross-references to general areas rather than specific headings are often in italic:
authors, see under the individual authors
In addition to inversion of proper names (see 19.3.2), wherever a heading (or subheading) consists of more than one word a decision must be made as to whether another heading in inverse form is also needed. As long as the first word is one that users will look for, the direct form of entry is better:
right of expression
secondary education
trial by jury
If the second and later words in the heading are also words that may be looked for, then additional inverse headings (and cross-references) can be made:
education, secondary
expression, right of
jury trials
Inverse headings need not be made automatically for every multiword heading, as applied consistently they can have a significant effect on the index length. The selection will depend on the context: for example, the heading education, secondary above is not needed if education is the subject of the whole text.
Terms such as names of organizations are often referred to in the text in an abbreviated form. The indexer must decide whether to make a heading under the shortened form or at the spelled-out version of the term. It is generally agreed that widely known terms such as UNICEF and NATO can be indexed in their shortened forms without cross references from the spelled-out forms. However, the indexer will need to decide how much cross-referencing is appropriate for the likely users of the index. It may be helpful to include the full form in parentheses after the shortened version:
OUP (Oxford University Press)
Oxford University Press, see OUP
A double entry may also be used:
OUP (Oxford University Press)
Oxford University Press (OUP)
although if space is an issue a cross-reference may be more economical. For reasons of neatness and apparent consistency, it will often be advisable to adopt a single practice (all entries cross-referred from shortened form to full title or vice versa; or all double entered) for all organization names in a given index.
The two systems of alphabetizing headings are word by word and letter by letter, with minor variations in each. The International and British Standard (BS ISO 999: 1996) advocates word-by-word indexing, which is the system usually employed in general indexes in Britain. Letter-by-letter indexing is preferred in British encyclopedias, atlases, gazetteers, and some dictionaries, and is more common in the US.
The word-by-word system alphabetizes compound terms (those that consist of more than one word or element) up to the first word space and then begins again, so separated words precede closed compounds (e.g. high water comes before highball). Hyphens are treated as spaces, and the two parts of a hyphenated compound are treated as separate words, except where the first element is not a word in its own right (e.g. de-emphasis, iso-osmotic, proto-language).
In the letter-by-letter system alphabetization proceeds across spaces, with separated (and also hyphenated) words being treated as one word.
In both systems the alphabetization ignores apostrophes, accents and diacritics. Parenthetical descriptions are also ignored: high (light headed) is treated as a simple heading (i.e. as high alone would be). Alphabetization continues until a comma indicates inverted order: for instance, High, J. is treated as High for alphabetization, although if there were several instances of High, the form High, J. would come after High alone and also after High, B. Bath, order of the would come before Bath bun and Bath chair.
Where headings are identical, indexing software will order terms in strict alphabetical order. This is preferred to some systems that order entries according to what they are, such as people first, then subjects, concepts, objects, places, and titles of works.
Thus in the example below High, J. comes first, as the name of a person.
These are time consuming because they require manual intervention on the indexer’s part and it can be argued that many index users will not be aware of these conventions.
The example below demonstrates alphabetization in the word-by-word and letter-by-letter systems:
| Word by word | Letter by letter |
|---|---|
| High, J. | High, J. |
| high (light-headed) | high (light-headed) |
| high chair | highball |
| high-fliers | highbrow |
| high heels | high chair |
| High-Smith, P. | Highclere Castle |
| high water | high-fliers |
| High Water (play) | high heels |
| highball | highlights |
| highbrow | Highsmith, A. |
| Highclere Castle | High-Smith, P. |
| highlights | high water |
| Highsmith, A. | High Water (play) |
| highways | highways |
In both systems, letter groups are treated as one word if—such as NATO and NASA— they are pronounced as such. Otherwise, the word-by-word system lists all sets of letters before any full word, ignoring any full points:
| Word by word | Letter by letter |
|---|---|
| I/O | I/O |
| IOU | iodine |
| IPA | IOU |
| i.p.i. | Iowa |
| IPM | IPA |
| i.p.s | IP address |
| IP address | Ipanema |
| Ipanema | i.p.i. |
| iodine | IPM |
| Iowa | i.p.s. |
Definite and indefinite articles at the beginning of entries are transposed in both systems:
Midsummer Night’s Dream, A
Vicar of Wakefield, The
In works written in English, foreign words are conventionally alphabetized by ignoring accents and diacritics, so for example ö and ø are treated as o. Some information on alphabetization in languages other than English is given in Chapter 12.
Personal names are generally given in inverted form to bring the significant element (the surname) forward: so Meynell, Alice rather than Alice Meynell.
Where people bear the same surname, initials are conventionally listed before full names; a name with a title that is otherwise identical with one without should follow it:
Meynell, A.
Meynell, Dr A.
Meynell, Alice
Meynell, F.
Meynell, Sir F.
Meynell, W.
Meynell, W. G.
List names prefixed with Mc, Mac, or Mc as if they were spelled Mac:
McCullers
MacFarlane
McFingal
McNamee
Personal names given only by surname in the text require a fuller form in the index, even if mentioned only in passing: Shepard’s illustrations is therefore expanded to the heading Shepard, E. H. Bare surnames should be avoided wherever possible: particularly for specialist subjects an author should anticipate inserting missing names in an index generated by an indexer, or checking for accuracy those the indexer supplies.
Personal names in a single numbered (usually chronological) sequence should be recorded in that sequence in spite of any surnames or other additions. Beware the omission of a number, especially of I; if others in the sequence appear duly numbered, restore the number when listing. Hence Frederick Barbarossa should become Frederick I Barbarossa and precede Frederick II. Where appropriate—especially for the period before c.1300—index people by their given names, with their titles, offices, etc. provided with suitable cross-references. Note again that descriptions in parentheses are disregarded for the purposes of alphabetization:
Henry
Henry (of France), archbishop of Reims
Henry, chaplain
Henry I, count of Champagne
Henry (the Lion), duke of Saxony
Henry, earl of Warwick
Henry II, emperor and king of Germany
Henry IV, emperor and king of Germany
Henry I, king of England
Henry II, king of England
Henry, king of England, the young king
Henry, scribe of Bury St Edmunds
Henry, son of John
Henry de Beaumont, bishop of Bayeux
Henry of Blois, bishop of Winchester
Henry Blund
Henry of Essex
Henry the Little
Henry de Mowbray
Henry Fitz Robert
Treat St as if it were spelled Saint, for both personal and place names. In alphabetical arrangement, saints considered in their own right as historical figures are indexed under their names, the abbreviation St being postponed:
Augustine, St, bishop of Hippo
Margaret, St, queen of Scotland
Rumwald, St, of Kings Sutton
When a place or a church is named after a saint, or the saint’s name complete with prefix is used as a surname, alphabetize it under the word Saint as if spelled out, not under St. Thus for example St Andrews, Fife, St Peter’s, Rome, and St John, Olivier are all treated as if they were written Saint —:
Saint, J. B.
St Andrews, Fife
St Benet’s Hall
St James Infirmary
St John-Smythe, Q,
Saint-Julien
St Just-in-Roseland
When the saint’s name is in a foreign language, alphabetize its abbreviation under the full form in that language: thus Ste-Foy is alphabetized as Sainte-Foy.
Foreign names are treated in the form familiar to the reader, so there is a comma in Bartók, Béla even though in Hungarian the surname comes first. Some information on alphabetizing non-English names is given at 6.1.8 and 6.1.9.
Alphabetize natural geographical features according to whether the descriptive component forms part of the name: Always retain the component if it is part of the official name:
Graian Alps
Grampians, the
Granby, Lake
Granby River
Gran Canaria
Grand, North Fork
Grand, South Fork
Grand Bérard, Mont
Grand Canyon
Grand Rapids
Grand Ruine, La
Grand Teton
Cape Canaveral
Cape Cod
Cape of Good Hope
Cape Horn
Where confusion may result—in atlases, for example—cross references or multiple entries are common.
If the first character or characters in a chemical compound is a prefix or numeral, such as O-, s-, cis-, it is ignored for alphabetizing but taken into account in ordering a group of similar entries. For example, ‘2,3-dihydroxybenzene’, ‘2,4-dihydroxybenzene’, and ‘cis-1,2-dimethyl cyclohexane’ would all be found under D, and the abbreviation ‘(Z,Z)-7,11-HDDA’, expanded as ‘cis-7,cis-11-hexadecadien-1-yl acetate’, would be alphabetized under H. In chemical notation disregard subscript numerals except when the formulas are otherwise the same:
vitamin B1
vitamin B2
vitamin B6
vitamin B12
Greek letters prefixing chemical terms, star names, etc. are customarily spelled out (and any hyphen dropped): for example, α Centauri, α chain, and α-iron are alphabetized as Alpha Centauri, alpha (α) chain, and alpha iron. However, Greek letters beginning the name of a chemical compound are ignored in alphabetization: for example, ‘α-oxo carbenes’ is spelled thus but alphabetized under O for oxo.
There are two systems for alphabetizing symbols and numerals: the International and British Standard advises listing them before the alphabetical sequence, but they are also commonly arranged as if spelled out, alphabetizing ‘=’ as equals, ‘£’ as pounds, ‘→’ as implies, ‘&’ as ampersand, ‘1st’ as first, and ‘7’ as seven. An ampersand within an entry is best treated either as if spelled out as and or ignored.
| Before the alphabetical sequence | As if spelled out |
|---|---|
| 1st Cavalry | 1st Cavalry |
| 2/4 time | 42nd Street |
| 3i plc | 3i plc |
| 42nd Street | 2/4 time |
Where the names of symbols may be problematic, it may be helpful to give an umbrella heading for symbols (for example rules of inference, linguistic symbols, coding notation) in addition to alphabetical listings. Whichever system is followed, maintain consistency throughout.
Arrangement of subentries should normally be alphabetical by key words (but see 19.4), ignoring leading prepositions, conjunctions, and articles in alphabetical ordering. Ensure that subentries are worded so that they are unambiguous and ‘read’ from or to the heading in a consistent pattern. Arrange subentries beneath related or similar headings in parallel. Cross-references from a main heading with subentries may either follow the locators for the main entry:
monasticism 20–3, 69, 131, 158, 202; see also ascetism; religious orders
cathedrals 112
churches 206
and mission 89, 90, 94, 134
reform 112–14
in Spain 287
or be given as the final subentry:
monasticism 20–3, 69, 131, 158, 202
cathedrals 112
churches 206
and mission 89, 90, 94,134
reform 112–14
in Spain 287
see also ascetism; religious orders
Some matter will call for ordering on some basis other than alphabetical, such as numerical, chronological, or hierarchical. Where this matter forms part only of an occasional group of subentries within an otherwise alphabetical index it may be ordered as necessary without comment; however, where a significant amount needs to be included in the form of headings it should be placed in a separate index where similar elements can be found and compared easily. Never arrange entries or subentries themselves by order of locator, as this is of least help to the reader.
Chronological ordering may be useful in arranging entries or subentries, for example according to the life and times of the subject in a biography:
dynasties, early:
Legendary Period (prehistoric) 1–33, 66, 178
Xia (c.2100–1600 BC) 35–60, 120
Shang (c.1600–c. 1027 BC) 61–84
Zhou (c.1027–256 BC) 12, 85–100, 178
Quin (221–207 BC) 109–35
Han (206 BC–AD 220) 132,136–7, 141
Hart, Horace:
birth 188
apprenticeship 192, 195
in London 195–9
in Oxford 200, 201–60
retirement 261
illness and death 208, 277–9
Subjects may sometimes be ordered according to some recognized hierarchical system of classification (e.g. BA, MA, MPhil, DPhil or duke, marquess, earl, viscount, baron). Indexes of scriptural references are arranged in their traditional order rather than alphabetically.
Index matter is set in small type, one or two sizes down from text size, usually set justified left (ragged right) in two or more columns. Typically, the running heads are Index on both recto and verso, though two or more indexes can be differentiated according to their title, such as Author index, General index, Index of first lines.
Begin each entry with a lower-case letter unless it is for a word that is capitalized in the text. Carefully check hyphenation, italics, spelling, and punctuation for consistency with the text. Instructions for cross-referring (see, see also) should be italicized. However, ‘see’ and ‘see also’ commonly appear in roman when they are followed by italicized text:
Plutarch’s Lives, see Parallel Lives; see also biographies; Dryden
Poema Morale, see Selections from Early Middle English
Poetics, see Aristotle
In Oxford style there is an en space between the entry and the first locator; interposing a comma is less usual nowadays. If a heading ends with a numeral (B-17, Channel 13, M25, uranium 235), a colon or comma can be added to separate it from the locator but a space is acceptable if that is the chosen style for other entries. Oxford style separates an entry from a following cross-reference with a comma but others omit the punctuation in accordance with other entries.
Separate multiple cross-references from each other with commas or semicolons:
earnings see income; taxation; wages
There is no punctuation at the end of entries. Oxford style inserts a colon after a main heading when there are no locators but instead a list of subentries
earnings:
income 12, 14–22, 45
taxation 9, 11, 44–9
wages 12–21, 48–50
but other styles only use the colon in this way in run-on entries but not in set-out style (see 15.5.2).
The first or only index in a work typically begins on a new recto, though subsequent indexes can begin on a new page.
The samples below show the two basic styles of typographic design for indexes, the subentries being either set out (or indented) or run on (or run in). The set-out style uses a new (indented) line for each sub entry; it is therefore clearer than the run-on style, though it takes up more room. In the set-out style, avoid further subdivision of subentries if possible, as this can result in complicated and space wasting structures. In the run-on style, subentries do as the name suggests: they run on and are separated from the main entry—and each other—by a semicolon. They are indented appropriately to distinguish them from the heading. Sub-subentries should be avoided entirely in run-on style. Take particular care that the arrangement is logical and consistent, since the style’s density makes it more difficult to read.
Set out |
Run on |
|---|---|
| shields 4, 78, 137, 140 heraldic designs 82 kite-shaped 199 round 195 Viking 43, 44, 53 |
shields 4, 78, 137, 140; heraldic designs 82; kite-shaped 199; round 195; Viking 43, 44, 53 |
|
ships/shipping 22, 68, 85, 230–52 design and navigation 6 pirate 23 spending on 59 see also galleys; longships; piracy |
ships/shipping 22, 68, 85, 230–52; design and navigation 6; pirate 23; spending on 59; see also galleys; longships; piracy |
| shipyards 234 | shipyards 234 |
Which style a publisher chooses depends on the length and number of subentries in the final index copy, and the conventions of related works. In any case, index copy must be submitted for setting with all entries and subentries in the set-out form for markup: it is easier for the typesetter to run these on afterwards, if necessary, than it is to set out an index from copy that was presented in the run-on format.
Turn-lines or turnovers (where text runs to more than one line of typescript) should be indented consistently throughout, and in set out style should be indented more deeply than the deepest subheading indentation. To save space, sub-subentries—where unavoidable—may be run on even in otherwise set-out indexes.
When an entry breaks across a page—most especially from the bottom of a recto to the top of a verso—the heading or subheading is repeated and a continuation note added during typesetting:
shields (cont.)
Viking 43, 44, 53
In references to pagination and dates, use the smallest number of figures consistent with clarity: see 11.1.4.
Be as specific as possible in your references. For this reason, do not use section or clause numbers instead of page numbers unless they are frequent and the entire index is to be organized that way. Avoid using ‘f.’ and ‘ff.’; give instead the first and last pages of the material: 123–5, for example, denotes one continuous discussion spanning three pages, whereas 123, 124, 125 denotes three separate short references. Avoid using passim (‘throughout’). Cite the page extent of a chapter and not, for example, Ch. 11.
References to footnotes and endnotes may simply give the page number. If more detail is desired, give references to footnotes and endnotes in the form ‘word 90 n. 17’ for one note and ‘word 90 nn. 17, 19’ for two or more; each has a full point and a space after the abbreviation. There is no need to give the note number where there is only one note on the page cited; in such cases it is Oxford style to insert a thin space between page number and ‘n.’
To provide the most effective help to the reader, a general index serving more than one volume must include the volume number as part of each locator, regardless of whether the pagination runs through volumes in a single sequence or begins anew with each volume. Volume numbers may be styled in Roman numerals, often in small capitals, separated by a full point: ‘word III. 90’. Indexes to a group of periodicals may have both the series and volume number as part of the locator.
Though simple page references are often sufficient, it may be appropriate to mark figures denoting references to illustrations in italic or bold, or with some typographic symbol (such as an asterisk or dagger), and provide an introductory note at the start of the index in the form Italic/bold numbers denote reference to illustrations. Some authors use a similar treatment to flag passages that are particularly significant or include definitions; again, explain this practice at the start of the index (but bear in mind that readers often ignore these explanatory notes).