Typographic Space
Typography is as much about shaping the space between and around letters, words, and blocks of text as it is about shaping type itself. In typography, there are terms for different kinds of spaces and there are principles that govern how these spaces should relate to one another. So let's take a closer look at typographic space.
The spacing of letters relative to one another in a typeface is called letter spacing. Type designers program letter spacing directly into the font file. In general, they aim to make the spaces between letters balanced with the spaces within letters.
Adding or subtracting space between the letters in a word, sentence, or paragraph in typesetting is called tracking.
One place for extra tracking is always appropriate is in setting all uppercase type. Because uppercase letters usually appear next to lower case letters. They're designed with the tighter spacing of the lower case. So when you set them with one another. You need to add tracking a good 20 to 50 units.
Adjusting the spacing between two particular letters regardless of the overall tracking is called kerning.
Kerning is sometimes necessary when you have two letters that fit together awkwardly. Typefaces these days are preprogrammed with kerning for problematic letter combinations. So manually kerning is rarely necessary. The space between lines of type in a paragraph. The space between one baseline and the next is called leading. The name comes from the strips of lead that used to be placed between lines of type in letterpress printing. A particular setting of type is described as a combination of type size and leading. A common setting for text in books, for instance is nine point type with 12 points of leading.
There's no easy rule to determine proper leading, but the principle is to set the lines of type far enough away from one another that our eyes can clearly distinguish between them, but still close enough together that we see the paragraph of type as a single block. And not as a collection of disconnected floating lines.
The x height of the typeface you're setting affects the leading, because typefaces with big x heights look visually larger than typefaces with small x heights, they need more leading.
Another factor in this matrix of spacial relationships, is line length. Line length is measured by the average number of letters that fit on a line. A good line length is long enough that your eye doesn't have to keep start new lines every few words, but short enough that when you come to the end of one line you're close enough to the start of the next to be able to find it easily. 66 characters per line is often seen as a sweet spot for line length, but up to 80 characters still works, and down to 40 for narrower columns of text also works.
So as you can see the variables that define typographic space, tracking, leading, line length, and alignment all effect one another. So as a typographer it's important to be really aware of all of these spacial relationships when your working with type.