In typography, letter-spacing, usually called tracking by typographers, refers to a consistent degree of increase (or sometimes decrease) of space between letters to affect density in a line or block of text.
Letter-spacing can be confused with kerning. Letter-spacing refers to the overall spacing of a word or block of text affecting its overall density and texture. Kerning is a term applied specifically to the spacing adjustment of two particular characters to correct for visually uneven spacing. In its original meaning with metal type, a kern meant having a letter stick out beyond the metal slug it was attached to, or cutting off part of the body of the slug to allow (other similarly-trimmed) letters to overlap. So a kern in that sense could only bring letters closer together (negative spacing), though of course it was possible to add space between letters. Digital kerning can go in either direction. Tracking can similarly go in either direction, though with metal type one could only adjust groups of letters further apart (positive spacing).
Letter-spacing adjustments are frequently used in news design. The speed with which pages must be built on deadline does not usually leave time to rewrite paragraphs that end in split words or that create orphans or widows. Letter-spacing is increased or decreased by modest (usually unnoticeable) amounts to fix these unattractive situations.
Digital systems of letter-spacing 
Varying applications 
Historically, personal computer based applications like Microsoft Word, Adobe Illustrator, QuarkXPress, Adobe InDesign, Adobe Photoshop, and Wordperfect, use differing, non-standard systems of adding or subtracting letter-spacing. What is common to most systems is that the default setting of letter-spacing or tracking is zero, using the widths (and kerning information) built into the font itself. Although digital type sets tighter on average than metal type, this is primarily a function of design decisions in the fonts, and the more ready availability of kerning, rather than any design choice inherent in the technology. In the days of machine-implemented lead typesetting (c.f. Linotype, Monotype), the amount of added spacing always had to be the same between each character. In today’s digital page-layout software, high-end applications all use relative measurements proportional to the size of the type. QuarkXPress uses units of 1/200 of the em, and the competing Adobe InDesign uses 1/1000 of an em. Thus in QuarkXPress a tracking setting of 3 opens text noticeably. However in the competing Adobe layout software product InDesign, a tracking setting of 3 would be barely noticeable.
Letter-spacing and legibility 
The amount of letter-spacing in text can affect legibility. Tight letter-spacing, particularly in small text sizes, can diminish legibility. The addition of minimal letter-spacing can often increase the legibility and readability. Added whitespace around the characters allows the individual characters to emerge and be recognized more quickly. (However, addition of space to the point that individual letters become isolated rather than simply easily identifiable destroys legibility and readability.) As reading with phonetic writing systems is based in part on context, and with unfamiliar words, on phonetic pronunciation, recognition of individual characters can be aided by slightly increased letter-spacing.
Effect on message 
The amount of letter-spacing can affect how text is perceived. Tight default letter-spacing, or negative letter-spacing, in text not only can reduce the legibility and readability of text, it can trigger a cultural association that tight letter-spacing is associated with advertising and therefore more subjective – the equivalent of a fast-talking car salesman. Conversely, the increase of letter-spacing in text (to an extent) increases legibility, and the cultural association is of a more objective typographic voice.
"Wide tracking" of text, beyond relaxed book composition, can look affected and earned the opprobrium of Frederic Goudy:
|“||Anyone who would letter space lower case would steal sheep.||”|
Until the advent of phototypesetting, the term "letterspacing" referred strictly to the adding of space between the individual letters of words set in metal type, in increments of a minimum of ½ point.
Letterspacing as such was expensive, involving the hand insertion of copper (½ pt.), brass (1 pt.), and printer's "lead" (2 pt.) spaces between individual pieces of type or between matrices on linecasting machines such as the Ludlow Typograph and the Linotype. As such, it was studiously avoided by compositors, as adding nothing more than time to an already laborious task.
The only exceptions were in advertising type or, in book work, in very short phrases in capitals or small capitals, to keep the phrases from being too visually black compared to the rest of the typographic composition.
Letter-spacing with fixed spaces 
Letter-spacing may also refer to the insertion of fixed spaces, as was commonly done in hand-set metal type to achieve letter-spacing. This is a more mechanical method which relies less upon spacing and kerning tables resident in each typeface and accessed and used when letterspacing is applied universally. Fixed spaces include a hair space, thin space, wordspace, en-space, and em-space. An en-space is equal to half the current point size, and an em-space is the same width as the current point size.
See also 
- Strizver, Ilene. "Top Ten Type Crimes". Fonts.com. Monotype Imaging. Retrieved 17 May 2011.
- Bringhurst, Robert. The Elements of Typographic Style. Hartley & Marks: 1992. ISBN 0-88179-033-8.
- Kane, John. A type primer. Prentice Hall: 2002. ISBN 0-13-099071-X.
- Lupton, Ellen. Thinking with Type: A Critical Guide for Designers, Writers, Editors, & Students. Princeton Architectural Press: 2007. ISBN 978-1-56898-448-3).
- Spiekermann, Erik. Stop Stealing Sheep & Find out how type works. Adobe Press: 2002. ISBN 0-201-70339-4.
- Owen Williams, Testing David. Nakai Theatre Home Grown Festival 2008, Whitehorse, Yukon, Canada, (2008)
- A logo with tight tracking Notice the letters almost touch each other, especially the “r” and “a”
- A logo with loose tracking Notice the large amount of white space between letters. An entire extra letter could fit in between each letter.