A ruler is an instrument used in geometry to measure short/medium distances and/or to rule straight lines. Strictly speaking, the ruler is the instrument used to rule and calibrated stick for measurement is called a measure. However, common usage is that a ruler is calibrated so that it can measure, creating ambiguity in what a ruler is allowed to do in Ruler-and-compass constructions. For instance, a ruler with the most rudimentary measurement capability (e.g. its own length) can be used for angle trisection. This is resolved by referring to an instrument that can only rule as a straightedge.
Practical rulers have distance markings along their edges. How these distance markings are applied and calibrated should be described here, including a history of old methods.