This is the bottom sheet used to fit tightly over a mattress. Fitted sheets are available in a variety of pocket depths, which refers to the thickness of the mattress. Standard North American pocket size is 7 to 9 inches. Deep pocket corners are usually 10 to 13 inches. Extra deep pocket corners are very generous in size, and range from 14” to as much as 22” and used for extraordinarily high/deep mattresses.