Within the key of C, the only legal chords are chords built off of the notes of the C major scale. This causes some chords to be major and some minor, because of the unequal spacing of tones in the scale. To build the standard three-note chord—a triadic chord—we start on any of the tones of the C major scale, skip one, and then use the next, then skip one again and use the next one after that. The first chord in C major, then, comes from the notes C-E-G, and because the first interval formed, between C and E, is a major third, we call this chord a major chord, and in particular, a C major chord. The next one we build in a similar fashion is composed of D-F-A. Because the interval between D and F is a minor third, this chord is called a D minor chord. Remember, major chords and minor chords have a very different sound. Even though most nonmusicians can’t name a chord on hearing it, or label it as major or minor, if they hear a major and minor chord back to back they can tell the difference. And their brains can certainly tell the difference—a number of studies have shown that nonmusicians produce different physiological responses to major versus minor chords, and major versus minor keys.
In the major scale, considering the triadic chords constructed in the standard way I’ve just described, three are major (on the first, fourth, and fifth scale degrees), three are minor (on the second, third, and sixth degrees) and one is called a diminished chord (on the seventh scale degree) and is made up of two intervals of a minor third. The reason we say that we’re in the key of C major, even though there are three minor chords in the key, is because the root chord—the chord that the music points to, the one that feels like “home”—is C major.
Generally, composers use chords to set a mood. The use of chords and the way they are strung together is called harmony. Another, perhaps better-known use of the word harmony is to indicate when two or more singers or instrumentalists are playing together and they’re not playing the same notes, but conceptually this is the same idea. Some chord sequences are used more than others, and can become typical of a particular genre. For example, the blues is defined by a particular chord sequence: a major chord on the first scale degree (written I major) followed by a major chord on the fourth scale degree (written IV major) followed by I major again, then V major, optionally to IV major, then back to I major. This is the standard blues progression, found in songs such as “Crossroads” (Robert Johnson, later covered by Cream), “Sweet Sixteen” by B. B. King, and “I Hear You Knockin’” (as recorded by Smiley Lewis, Big Joe Turner, Screamin’ Jay Hawkins, and Dave Edmunds). The blues progression—either verbatim or with some variations—is the basis for rock and roll music, and is found in thousands of songs including “Tutti Frutti” by Little Richard, “Rock and Roll Music” by Chuck Berry, “Kansas City,” by Wilbert Marrison, “Rock and Roll” by Led Zeppelin, “Jet Airliner” by the Steve Miller Band (which is surprisingly similar to “Crossroads”), and “Get Back” by the Beatles. Jazz artists such as Miles Davis and progressive rock artists like Steely Dan have written dozens of songs that are inspired by this progression, with their own creative ways of substituting more exotic chords for the standard three; but they are still blues progressions, even when dressed up in fancier chords.
Bebop music leaned heavily on a particular progression originally written by George Gershwin for the song “I’ve Got Rhythm.” In the key of C, the basic chords would be:
C major–A minor–D minor–G7–C major–A minor–D minor–G7
C major–C7–F major–F minor–C major–G7–C major
C major–A minor–D minor–G7–C major–A minor–D minor–G7
C major–C7–F–F minor–C major–G7–C major
The 7 next to a note name indicates a tetrad—a four-note chord—that is simply a major chord with a fourth note added on top; the top note is a minor third above the third note of the chord. The chord G7 is called either “G seven” or “G dominant seven.” Once we start using tetrads instead of triads for chords, a great deal of rich tonal variation is possible. Rock and blues tend to use only the dominant seven, but there are two other types of “seven” chords in common use, each conveying a different emotional flavor. “Tin Man” and “Sister Golden Hair” by the group America use the major seven chord to give them their characteristic sound (a major triad with a major third on top, rather than the minor third of the chord we’re calling the dominant seven); “The Thrill Is Gone” by B. B. King uses minor seven chords throughout (a minor triad with a minor third on top).
The dominant seven chord occurs naturally—that is, diatonically—when it starts on the fifth degree of the major scale. In the key of C, then, G7 can be constructed by playing all white notes. The dominant seven contains that formerly banned interval, the tritone, and it is the only chord in a key that does. The tritone is harmonically the most unstable interval we have in Western music, and so it carries with it a very strong perceptual urge to resolve. Because the dominant seven chord also contains the most unstable scale tone—the seventh degree (B in the key of C)—the chord “wants to” resolve back to C, the root. It is for this reason that the dominant seven chord built on the fifth degree of a major scale—the V7 chord, or G7 in the key of C—is the most typical, standard, and clichéd chord right before a composition ends on its root. In other words, the combination of G7 to C major (or their equivalents in other keys) gives us the single most unstable chord followed by the single most stable chord; it gives us the maximum feeling of tension and resolution that we can have. At the end of some of Beethoven’s symphonies, when the ending seems to go on and on and on, what the maestro is doing is giving us that two-chord progression over and over and over again until the piece finally resolves on the root.