A ruler is an instrument used to measure distances, degrees and determine straight lines. There are many different types of measuring rulers, available in different shapes, sizes and materials. These rulers are used in various occupations and fields of study, such as geometry, printing, technical drawing, building and engineering.

### Desk Ruler

The best-known and most common type of ruler, desk rulers are generally used for measuring, drawing straight lines and as guides when cutting with blades. They have measurement markings along each side; generally, inches are marked on one side and centimetres are marked on the other. Standard desk rulers measure 12 inches or 30 centimetres long. They can be manufactured from a number of different materials, such as wood, plastic or metal, and are available in clear or opaque varieties.

### Line Gauge

Line gauges are rulers used by printers that resemble desk rulers, but are marked in points, picas, agates and inches. Sometimes centimetres are also included. Line gauges are made of stainless steel and have engraved markings. They are generally 72 picas or 11.952 inches long.

### Protractor

Used to measure angles or circles, protractors are clear, circular or half-circular rulers. They come in two varieties: 180 degrees -- half-circle -- or 360 degrees -- full circle. To measure an angle, protractors have two sets of degree units running along the curve, going in opposite directions. These degree units are usually marked off in half-degree increments. Protractors are available in three sizes: six inches, eight inches or 10 inches.

### Drafting Triangle

Drafting triangles, usually made of clear plastic, are used to measure angles and draw vertical or inclined lines. The two types available, the 45-degree triangle and the 30/60-degree triangle, are named by the size of their acute, or small, angles. The 45-degree triangle has a 90-degree angle and two 45-degree angles; the 30/60 degree triangle has a 90-, a 30- and a 60-degree angle. Drafting triangles are available in sizes ranging from four to 18 inches.

### Adjustable Triangle

Combining the functions of protractors and drafting triangles, adjustable triangles can be set and locked at any angle. The degrees of an angle, marked in half-degree increments, are found on the protractor portion and then locked in place. The triangle portion can then be used to draw the selected angle.

### Triangular Scale Ruler

Triangular scale rulers, scale-measuring devices that are triangular in shape, are used by architects, engineers, builders and designers. They are marked with a range of scales or ratios, used for making or measuring reduced-scale drawings like blueprints or floor plans. In the United States, the ratios on these rulers are usually marked as inches-to-the-foot. In other parts of the world, the ratios are marked in metric terms, such as millimetres, centimetres and meters. Scale rulers generally have 10 to 12 scales available and are 12 inches in length.