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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

The word straightedge can also be rendered as straight edge.

Some sources hyphenate: 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

  • Results about straightedges can be found here.