# Definition:Straightedge

## Contents

## 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 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.

## Also see

## Sources

- 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