A ruler is often used to measure length. Length is a two-dimensional measurement as to how long an object is, how tall an object is or how far apart objects are. The most common units used to measure length include feet, yards, miles, centimeters, meters or kilometers.
There are generally two systems used in measurements, the standard and metric unit systems. In the standard unit system, one foot is equal to 12 inches. Another common conversion of length measurement is that one yard equals three feet, which also equals 36 inches. In the metric system, the meter is the basis of the length measurement system. Everything is based upon the meter; for example, one centimeter equals 10 millimeters.