30c Use a comma between all items in a series.

In a series of three or more items (words, phrases, or clauses), use a comma between all items, including the last two.

Although some writers view the last comma in a series as optional, most experts advise using the comma because its omission can result in ambiguity or misreading.

The example reads,

My uncle willed me all of his property, houses (insert) comma (end inserts) and boats.

Did the uncle will his property and houses and boats m dash or simply his property, consisting of houses and boats? If the former meaning is intended, a comma is necessary to prevent ambiguity.