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### Length Values

A length value is expressed as a positive or negative number (when permitted), followed immediately by a two-letter abbreviation that represents the units to be used. There should never be any space between the number and the unit designator. Note that a value of `0` (zero) need not have a unit designator. Length units are divided into two types: absolute units, which are (in theory) always measured in the same way, and relative units, which are measured in relation to other things.

#### Absolute length units

Inches (`in`)

As you might expect, the same inches found on typical U.S. rulers. The mapping from inches to a monitor or other display device is usually approximate at best because many systems have no concept of the relation of their display areas to “real-world” measurements such as inches. Thus, inches should be used with extreme caution in screen design.

Centimeters (`cm`)

The centimeters found on rulers the world over. There are 2.54 cm to an inch, and 1 centimeter equals 0.394 inches. The same mapping warnings that applied to inches also apply to centimeters.

Millimeters (`mm`)

There are 10 millimeters to a centimeter, so you get 25.4 mm to an inch, and 1 millimeter equals 0.0394 inches. Bear in mind the previous warnings about mapping lengths to monitors.

Points (`pt`)

Points are standard typographical measures used by printers and typesetters for decades and by word-processing programs for many years. By modern definition, there are 72 points to an inch. Therefore, the capital ...

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