A ruler, also known as a rule, is a flat, straightedge measuring instrument with equally marked spacing etched or moulded along its length. The spacing forms a measuring scale of either metric or imperial measurements, which are usually in increments of centimetres (cm) and millimetres (mm) and inches (sometimes denoted as ") respectively. Rulers are used to measure the length of an object or its edge precisely and as a guide for drawing or scoring straight lines. Some rulers have measurements on both sides. The edges of rulers are often bevelled to reduce measurement errors.
The better question may be 'who doesn't?' Rulers are used everywhere, from schools and offices to within the home and in a wide range of occupations. They are prevalent is particular fields of study including geometry, printing, technical drawing, architecture, building and engineering.
Rulers are made from durable materials like wood, plastic or metal. These materials are hard wearing and are usually shatter resistant and scratch proof as they are made to last a long time.