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Points are the basic unit for measure for typefaces. However, the same size type characters look very different from one typeface to another. The thousands of font styles available each have unique qualities of shape, weight and proportion that affect their appearance.
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Identifying Character Features
To understand measuring the point size of type, let’s look at character structure. Letters of the alphabet include capitals and tall letters, shorter lower case letters, and some with tails that hang below the "baseline", where the line of type sits.
The "cap height" is measured from the baseline to the top of capital letters. "X height" is from the baseline to the top of most lower case letters, called the "median line". The "ascender line" rests at the top of the capitals. The "descender line" marks the lowest point of letters with tails. The full range of height - from descender line to ascender line - is the "body size" of the font.
The point size of a font is determined by measuring its body size. The distance between lines of type, called leading, is also determined by measuring in points. Different fonts with the same body size may appear as different sizes because of variation in x height.
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Understanding the Point System
Type is measured by its body size using points because of its history. In early letterpress days, type characters were physically mounted onto individual blocks of wood. Type was a physical object cast by a foundry. Its body height was the minimum amount of space required to insert a character. Each typeface was available in a limited number of sizes. The cost of purchasing new fonts was prohibitive.
The point system (the Didot point) was commonly used by the printing industry in the 1700s, so it was quickly adopted for measuring type. The measurement was based upon the French royal foot and was equal to .351 mm. Twelve points equal a pica, with six picas to the inch. Of course, the French foot and the American foot are not exactly the same size. When dealing with such small increments as points, it does allow confusion.
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Digital Typography and Points
Today, points are standardized to 72 points per American inch (approximately .353 mm), making measuring the point size of type more consistent. This change occurred in 1984 when Apple Computer introduced the first Macintosh with 72 pixel per inch monitors that displayed PostScript typefaces in actual size and shape (What you see is what you get technology - WYSIWYG). The point system we use for digital typography is known as the "PostScript point." PostScript technology, which allows for scalable fonts with curve, was developed by Adobe Systems.
The world of typography has changed forever because of computer technology. Desktop publishers are free to use any point size of type, even mixing sizes and styles for single letters, if desired.