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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


$(2): \quad$ 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