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134 Chapter 3: Elements of Style Points and picas are traditional typographic measures. There are 12 points to a pica and 6 picas to an inch. Therefore, a font size of 18 points is equivalent to 1.5 picas, or .25 inches. Ems and exes are also typographic measures. Like a percentage value, they specify sizes relative to the element’s computed size rather than an absolute number of units. When type was set using individual letters cast in lead alloy, font sets included spacer characters of different sizes. An em-quad was a square space just large enough to contain an uppercase M. Exes. | 134 Chapter 3 Elements of StyLE Points and picas are traditional typographic measures. There are 12 points to a pica and 6 picas to an inch. Therefore a font size of 18 points is equivalent to 1.5 picas or .25 inches. Ems and exes are also typographic measures. Like a percentage value they specify sizes relative to the element s computed size rather than an absolute number of units. When type was set using individual letters cast in lead alloy font sets included spacer characters of different sizes. An em-quad was a square space just large enough to contain an uppercase M. Exes are squares the size of a lowercase x. Obviously a larger font will have larger ems and exes in absolute terms. However in CSS one em is equivalent to 100 percent of the current font size. To illustrate consider the following three CSS statements main main p.first main p.second font-size medium font-size 1em font-size 100 In a division element with id main the two nested paragraphs with classes first and second will have the same font size measured in pixels if they are in the same font family. If the two paragraphs are of different font families they may appear to have different font sizes. An em measure is only a guideline for font designers who are free to make their letters larger or smaller as they please. Here are some examples blockquote font-size 18pt p.event font-size 16px intro font-size 7mm address font-size 1.75em Font sizes given in pixels are relative to the browser s viewport and therefore are dependent on the resolution of the reader s display device and the viewer s distance from the display. Pixels are not simple screen dots anymore. However as a rough guide a 17-inch monitor measured diagonally set to a resolution of 1024 by 768 pixels is very close to 72 pixels per inch. Because this is now considered the low end of display screen technology most PC users are looking at a screen with approximately 96 pixels per inch. Mobile and tablet users may have resolutions and .