Type is defined by the space around it, whether between letters, words, or lines.
Typewriter typefaces are usually fixed pitch. Fixed pitch means that each character, whether it's an 'i' or an 'm,' takes up the same amount of space. A fixed-pitch typeface, such as Courier, works well with the simple, mechanical design of a typewriter.
Commercially-printed text and all the modern digital type used on computers is generally designed to be proportionally spaced. With proportional spacing, each letter is given just the amount of space it needs to look right and be most legible. Using a proportional typeface, you can fit much more text on a page than using a fixed-pitch face and can improve readability.
As lines of text get long, it can be difficult for the reader to move from the end of one line to the beginning of the next. On the other hand, short line lengths break up the text and interrupt the reader. The ideal line length depends on the design of the typeface, type size, line spacing, and length of the copy. Generally, a line should have 55 to 60 characters, or 9 to 10 words, for optimal readability.
Leading is the distance between lines of type and is measured in points. During the days of metal type, printers inserted extra strips of lead between long lines of text to make them easier to read. This procedure gave rise to the term "leading." Most word processing and page layout applications let you adjust the leading in your documents. Experiment with this feature to see how it affects legibility.
You can also adjust word spacing and letterspacing to improve legibility. Although typefaces are designed with the correct spacing between characters for general use, special situations can result in the type's looking crowded or too loose. For example, words printed in all UPPERCASE tend to look too tight because the designer assumed that uppercase and lowercase letters would be mixed. If your application allows you to adjust letterspacing, you should add a small amount of letterspace to words printed in all uppercase.
Many letter combinations, particularly in words set in capitals, do not look right together unless they are kerned. Kerning is the adjustment of space between pairs of letters. Kerning is especially important at large point sizes. As the characters are enlarged, so is the space between them.
Word spacing, the space between words, should be constant in flush left, flush right, or centered text. However, for justified text, word spacing varies from line to line to keep margins even, and it is important to keep word spacing as consistent as possible, often with the use of hyphenation to aid readability. Tight word spacing lets you place more text on the page, but can make it difficult to distinguish words from each other. Loose word spacing fills up a page with a small amount of text, but becomes harder to read as the words begin to look disconnected.
Spacing concerns and the design of the typeface itself affect what is known as typographic color. This term may seem like a misnomer in an age when even word processors let you apply actual color (for example, red, blue or green) to type as easily as changing the point size. Typographic color is really the gray value, or density, of a mass of type on the page. A page may have light or dark color, but you must keep the color consistent on the page to aid readability.