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.