Definition:Straightedge

Definition
A straightedge is an ideal tool for constructing straight lines.

A straightedge is of unlimited length, but has no markings on it, so it cannot be used for measurement.

Hence it can be used either:
 * $(1): \quad$ to construct a line segment between two given points, according to Euclid's first postulate

or:
 * $(2): \quad$ to extend a line segment in either direction indefinitely, according to Euclid's second postulate.

Also see

 * Definition:Compass and Straightedge Construction