The term " minute " usually means 1 ⁄ 60 of an hour, coming from "a minute division of an hour". The term " second " comes from "the second minute division of an hour", as it is 1 ⁄ 60 of a minute, or 1 ⁄ 60 of 1 ⁄ 60 of an hour. While usually sub-second units are represented with SI prefixes on the second (e.g. milliseconds), this system can be extrapolated further, such that a "Third" would mean 1 ⁄ 60 of a second (16.7 milliseconds), and a "Fourth" would mean 1 ⁄ 60 of a third (278 microseconds), etc. These units are occasionally used in astronomy to denote angles.