The comma’s ancestors have been used since Ancient Greece, but the modern comma descended directly from Italian printer Aldus Manutius. (He’s also responsible for italics and the semicolon!) In the late 1400s when Manutius was working, a slash mark (/, also called a virgule) denoted a pause in speech. (Virgule is still the word for comma in French.) Manutius made the slash lower in relation to the line of text and curved it slightly. In the 1500s, this new mark acquired the old Greek name “comma”. The word comma literally meant “a piece cut off” from the Greek word koptein meaning ”to cut off”.
Other than the period, the comma is the most common punctuation mark in English, but the little mark is often misunderstood and misused, even by native speakers. The comma plays an important role in the sentence because it tells a reader when to pause briefly. When should the comma be used? The comma is often used to separate items in a list as in the sentence: “Mark went to the store to buy eggs, bread, milk, and blueberries.” The third comma in that sentence is the topic of much debate. That comma—before “and”—is called the “serial comma” or the “series comma”. Some usage conventions require a serial comma, but others do not. Whether or not to use a serial comma can also depend on the items in the list. Consider this sentence: “For breakfast, Mary had an apple, toast and jam, and coffee.” Without the final comma the sentence would be unclear. She isn’t eating jam with coffee, so a serial comma clarifies the situation.
Commas are also used to separate independent clauses when a conjunction is used, as in the compound sentence: “Mark went to the store, and he bought eggs, bread, milk, and blueberries.” Commas have many other uses as well. When an entire phrase may be removed from a sentence, commas are used to set the phrase apart. Take this as an example: “Shelia, reconsidering her options, did not want to go to the movie.” In a similar fashion, they set off introductory participle phrases as in: “Reading over her notes, Julie realized she missed an important detail.”