A ruler is a person in charge of a country. See Monarch, Lists of incumbents

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 the the 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.

A ruler is also a popular tool used for corporal punishment, usually on the hands or thighs. Generally, wooden rulers are used for this purpose, but plastic ones can also be used in spanking.

The ruler (calibrated, though numbers are not shown) appears as a charge in heraldry in the arms of Odouze.

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