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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 to the measuring edge) and little lines (perpendicular 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.
- Parallel means extending in the same direction.
- Perpendicular means at right angles.