A ruler is an instrument used in geometry to measure short/medium distances. However, the ruler in Ruler-and-compass constructions is really 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.
Pictures and different style of rulers should be added later. Right now, this article is just a stub.
wikipedia.org dumped 2003-03-17 with terodump