Commas and periods are the most frequently used punctuation marks. Commas customarily indicate a brief pause; they're not as final as periods. Rule 1. Use commas to separate words and word groups in a simple series of three or more items. Example: My estate goes to my husband, son, daughter-in-law, and nephew. Note: When the last comma in a series comes before and or or (after daughter-in-law in the above example), it is known as the Oxford comma.