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
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