Someone who exercises supreme authority or control over a state. The term was traditionally used with reference to individuals who have a hereditary right to rule (eg kings, queens, shahs), but is now also applied to the leaders of elected governments (eg presidents, prime ministers) and to those who have seized power (eg dictators, military juntas). In many instances, different notions of rule co-exist, as in the case of the UK, where both the monarch and the political party in government can be said to rule.
A ruler or rule is an instrument used in geometry, technical drawing and engineering/building to measure distances and/or to rule straight lines. Strictly speaking, the ruler is the instrument used to rule lines and the calibrated instrument used for determining measurement is called a measure.
Several different designs of flexible instruments are used to determine length, such as the carpenter's rule, the ribbon-like tape measure used by tailors, and the retractable rule used especially in the construction trades and by home handyman, also known as a tape measure. As can be seen by the photos on this page, a 2 metre carpenter's rule can be folded down to a length of only 20 centimetres to easily fit in a pocket, and the 5 metre long tape easily retracts to fit within a small-sized housing.
Geometric constructions
In geometry, a ruler usually refers to an unmarked ruler, only used for drawing straight lines between points, not measuring. A ruler and compass construction thus refers to constructions using such unmarked ruler and a compass.
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