**Twelve millimeters is equal to 0.472 inch.** An inch is defined as exactly 25.4 millimeters by the system of imperial units as well as the system of U.S. customary units.

The word "inch" comes from the Latin word "uncia," which means one-twelfth part. The word "ounce" also originated from the same word. An inch is abbreviated as "in.", and is denoted by either "in." or a double prime. A double prime is approximated by a double quotation mark, which means that 5 inches can be written as 5".

There are 12 inches in a foot, and feet are denoted by a single prime, approximated by an apostrophe. So if a person is 5 feet 6 inches tall, it's written as 5'6". The inch was first defined by the British Weights and Measures Act of 1824, and it was used as a part of a system of units that are known as the imperial system. The imperial system originated from the pre-1824 system known as "English units."

Although the British Weights and Measures Act of 1824 was scheduled to go into effect May 1, 1825, it was not implemented until Jan. 1, 1826. Since 1824, it has been refined and reduced several times.