Making a plural of most words is straightforward--simply add an "s." But it can be tricky to make a plural of a word that ends in an "s" sound to begin with. This type of sound, called a sibilant, is produced when a word ends in "s," "sh," "ch," "z," or "x." To pluralize such a word, "es" should be added to the end. This seems obvious for most nouns, but people generally get confused when pluralizing names that end with a sibilant. In these cases, "es" at the end is the proper form.
For example, the plural of Williams is Williamses, the plural of Knox is Knoxes, and the plural of Jones is Joneses.
Take note of one special exception for names that end in "y." Instead of changing the "y" to an "i" and adding "es," the name keeps the "y" at the end and simply adds an "s." For example, the plural of Marley is Marleys and the plural of Berry is Berrys.