Ruler
A ruler (or 'rule') is an instrument of measurement. Rulers measure length. Length is how long or short something is. Most rulers work by having numbers (parallel[1] to the measuring edge) and little lines (perpendicular[2] to the measuring edge). Rulers can also be used as a straight edge to draw lines with.
Rulers come in many different forms. They can be made of plastic, wood and metal, and other materials. Rulers also come in a variety of lengths. For example, there are 1 metre rulers, 30 centimetre rulers, 20 centimetre rulers and 15 centimetre rulers. These are examples of metric rulers. You can also get 1 yard rulers, 18 inch rulers and 6 inch rulers, etc. These are examples of imperial rulers.