Rulers

A ruler is an instrument used to make length measurements, whereby a length is read from a series of markings called "rules" along an edge of the device. Alternatively, it is called a rule, scale, line gauge, or metre/meter stick. Usually, the instrument is rigid and the edge itself is a straightedge ("ruled straightedge"), which additionally allows one to draw straighter lines. Rulers are an important tool in geometry, geography and mathematics. They have been used since at least 2650 BC.