HTML and CSS Reference
In-Depth Information
Length Values
A length value is expressed as a positive or negative number (when permitted), followed im-
mediately 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. A value of 0 (zero) does not require
a unit designator. Length units are divided into two types: absoluteunits, which are (in theory)
always measured in the same way, and relativeunits, which are measured in relation to other
Absolute Length Units
Inches( iin )
As you might expect, the same inches found on typical US 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” measure-
ments such as inches. Thus, inches should be used with extreme caution in screen design.
Centimeters( ccm )
The centimeters found on rulers the world over. There are 2.54 centimeters to an inch, and
1 centimeter equals 0.394 inches. The same mapping warnings that applied to inches also
apply to centimeters.
Millimeters( mmm )
There are 10 millimeters to a centimeter, so you get 25.4 millimeters to an inch, and 1 mil-
limeter equals 0.0394 inches. Bear in mind the previous warnings about mapping lengths
to monitors.
Points( ppt )
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 letters of text set to 12 points should be one-sixth
of an inch tall. For example, p {font-size: 18pt;} is equivalent to p {font-size:
0.25in;} , assuming proper mapping of lengths to the display environment (see previous
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