When three or more items are presented in a series, those items should be separated from one another with commas. Items in a series may be single words, phrases, or clauses.

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.

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