A ruler is a tool which can be used for measuring a distance or to draw 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 is 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. The difference between the measures on the ruler is the distance between the 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.

When using a ruler to draw a segment between two points, the ruler is laid with its edge intersecting both points. The segment is then drawn by running a pen or a pencil along the edge of the ruler.

A ruler that does not have any length scale printed on it is referred to as a straightedge. Straightedges can be used to draw lines but is of limited use when it comes to measuring distances.