直尺也有人称为间尺，是一种非常常用的计量长度仪器，这种文具极为普遍，几乎每位小学生都人手一把，通常用于量度较短的距离或画出直线。现代的直尺则多与三角尺、量角器、圆规等成套装出售。

直尺的发明己不可考究，坊间依稀传着直尺和圆规是由伏羲及女娲发明的。山东东汉武梁祠石室留有「伏羲氏手执矩，女娲氏手执规」的造像，其中的「矩」所指的便是「直尺」了。

• 数学王子高斯曾于19岁时只用圆规和直尺画出正十七边形。

A straightedge is a tool with an edge free from curves, or straight, used for transcribing straight lines, or checking the straightness of lines. If it has equally spaced markings along its length, it is usually called a ruler.

Straightedges are used in the automotive service and machining industry to check the flatness of machined mating surfaces.