# What Are Some Things That Come in Groups of Ten?

Things often found in groups of 10 include human fingers and toes and the number of digits in a full phone number. There are 10 years in a decade. It is theorized that the decimal system, also known as base-10 mathematics, developed as a result of humans having 10 fingers.

10-digit phone numbers are increasingly common in metropolitan areas of the United States and Canada. Population growth and the growing use of cell phones has increased the need for new phone numbers. To meet this demand, area code districts have been divided and a different code is given to new districts. A seven-digit phone number can then be used twice in the original district, with one number being given the new three-digit area code.

A decade is defined as any period of 10 years in consecutive order. Decades are typically thought of as blocks of time in which the years all have the same next-to-last digit, such as 1980 through 1989. However, a decade can start at any point of time. For example, the phrase "for the past decade" indicates the 10 years prior to the current year.

The Roman numeral for 10 is the letter X. To "decimate" something in Roman times meant to reduce it by one-tenth.