A ruler, a short length of wood, plastic or metal, is used to find out how long something is. In the United States, the units of measurement are inches, feet and yards. In the United States most rulers are 12 inches (30.48 centimeters) long and have big marks to show inches and smaller marks for parts of an inch. In much of the world the metric system is used, which measures in millimeters, centimeters and meters. The United Kingdom uses both systems of measurement and many rulers have inches on one side and centimeters on the other side.

Here's how to use and read a ruler.

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Materials needed:

- 12 inch ruler
- 30 centimeter ruler
- Pencil
- Paper

First we'll measure a line in inches:

- Make a straight horizontal line on the sheet of paper, using the pencil and the 12-inch ruler. The line should be shorter than the ruler.
- Place the ruler along the line. The edge of the ruler with the lower numbers should be at the left end of the line. The ruler should span the entire length of the line. The line will end near one of the marks on the ruler.
- Look at the big mark closest to the right of the end of the line. That will give you a number in whole inches.
- Count how many smaller marks there are between this big mark and the end of your line. Each small mark is ? of an inch. If your line ends one small mark after the 7-inch mark, the line measures 7? inches [source: Conversion, Lesson Snips].

Now we'll measure a line in centimeters

- Repeat steps 1 and 2 above, using the 30-centimeter ruler.
- Look at the big mark closest to the right of the end of the line. That will give you a number in whole centimeters.
- Count how many small marks there are between the big mark to the left and the end of your line. Each small mark is one millimeter. If your line ends one small mark after the 7 centimeter mark, the line measures 7 centimeters and 1 millimeter, or 7.1 centimeters [source: Conversion, Lesson Snips].

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