A ruler is a geometric tool that can be used for measuring distances or drawing segments. Rulers are typically made of plastic, wood, or steel, but other materials may also be used.

To use a ruler to measure a distance, one or more length scales are printed on the ruler. Rulers come in various sizes, lengths, and shapes. Many rulers come in the shape of a rectangle, usually with a length 3 to 15 times greater than its width. The ruler depicted below has a length 7 times greater than its width.

When a ruler is used for measuring a distance between two points, the ruler is placed with its edge intersecting both points. For many measurements, the ruler can be placed so that the edge intersects one of the points at the measure 0 on the ruler. The distance can then be read directly on the measuring scale.

Sometimes, due to physical restrictions, it may be impossible to measure from 0 on the ruler. In such cases, the difference between two measures on the ruler can be calculated to find the distance between the points.

A ruler can be used as a straightedge to draw a segment between two points. The ruler is laid with its edge intersecting both points and the segment is drawn by running a pen or a pencil along the edge of the ruler.