A straightedge with markings to indicate distances. Although geometric constructions are sometimes said to be performed with a ruler and compass, the term straightedge is preferable to ruler since markings are not allowed by the classical Greek rules.
Ruler
See also
Coastline Paradox, Compass, Geometric Construction, Geometrography, Golomb Ruler, Perfect Ruler, Simplicity, Slide Rule, StraightedgeExplore with Wolfram|Alpha
References
Gardner, M. Wheels, Life, and Other Mathematical Amusements. New York: W. H. Freeman, pp. 153-155, 1983.Gardner, M. The Magic Numbers of Dr Matrix. Buffalo, NY: Prometheus, p. 230, 1985.Golomb, S. W. "How to Number a Graph." In Graph Theory and Computing (Ed. R. C. Read). New York: Academic Press, pp. 23-37, 1972.Smogorzhevskii, A. S. The Ruler in Geometrical Constructions. New York: Blaisdell, 1961.Referenced on Wolfram|Alpha
RulerCite this as:
Weisstein, Eric W. "Ruler." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Ruler.html