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
- $(2): \quad$ to extend a line segment in either direction indefinitely, according to Euclid's second postulate.
Also known as
This can also be rendered as straight edge.
Some sources use the term ruler, but this is inaccurate as a ruler is generally understood to have scale markings on it.
- 1969: C.R.J. Clapham: Introduction to Abstract Algebra ... (previous) ... (next): Chapter $8$: Field Extensions: $\S 40$. Construction with Ruler and Compasses
- 2008: Ian Stewart: Taming the Infinite ... (previous) ... (next): Chapter $2$: The Logic of Shape: Problems for the Greeks
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): Entry: straight edge