When you need to measure an object's dimensions, such as a section of cardboard that you want to cut to a specific length and width, you can use a ruler to find its size. If your task requires you to use measurement units typically employed in the United States, you will use a regular ruler that has feet and inches. If you need to make measurements in the metric system, you will use a metric ruler that has meters and centimeters.
About Rulers

A ruler is a long, thin device that you use to measure the length of an object. It might be made of wood, metal or plastic. It is flat and marked with numbers at regular intervals, so you can hold it against something and match the item to the length marked on the ruler. You can place a ruler against a wall and mark off in feet and inches, or meters and centimeters; for example, have a child stand against the wall to mark her development as she grows in height over the years.
About the Metric System

Developed in France in the 1790s, the metric system is the dominant measurement system in every industrialized country except the United States, according to the U.S. National Institute of Standards and Technology. While the system used in the United States divides a foot into 12 units called inches, which are further divided into quarterinches, the metric system is based on powers of 10. A meter is divided into 100 centimeters, and a centimeter is further divided into millimeters. There are 5,280 feet in a mile, and 1,000 meters in a kilometer. The United States designates the metric system as the preferred system of weights and measures, but this is a recommendation, not a mandate. Familiar uses of the metric system in the United States include 2liter soda bottles and 35mm camera film.
Feet and Inches, Meters and Centimeters

On a regular ruler, you will typically see 1 to 3 feet in length. Each foot contains 12 inches, so in a 3foot ruler, there are a total of 36 inches. In comparison, a metric ruler may typically have a single meter divided into 100 centimeters. One meter equals 3.28 feet, and 1 inch equals 2.54 centimeters.
Fractional Measurement

When you need to make measurements using figures that are more precise than an inch or a centimeter, you will use fractions of these units of measure. On a ruler that uses feet and inches, you will find smaller divisions, so you can measure the length of something as being 1 foot, 4 1/4 inches. With a metric ruler that uses meters and centimeters, you will see smaller fractions in millimeters, so you can measure something that is 11 mm long, for example.
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