In typography, letter-spacing or tracking is an optically-consistent adjustment to the space between letters to change the visual density of a line or block of text. Letter-spacing is distinct from kerning, which adjusts the spacing of particular pairs of adjacent characters which would appear to be badly spaced if left unadjusted.
Historically, with metal type, a kern meant having a letter stick out beyond the metal slug to which it was attached, or having part of the body of the slug cut off to allow letters to overlap. A kern could therefore only bring letters only closer together (negative spacing). Digital kerning could go in either direction. Tracking can similarly go in either direction, but with metal type, one could make groups of letters only farther apart (positive spacing).
In the days of hot metal typesetting, letter-spacing required adding horizontal space between letters of words set in metal type in increments of a minimum of a half-point. Some publishers and typesetters avoided letter-spacing because it was costly in materials and labor. Letter-spacing required hand insertion of copper (a half-point), brass (one point), and printer's "lead" (two points) spaces between individual pieces of type or between matrices. Despite the cost, letter-spacing was used in print advertising and book publishing. It was also used for very short phrases set in capital letters or small caps to prevent the phrases from appearing too black compared to the rest of the page.
Printer and type designer Frederic Goudy stated that "Men who would letterspace blackletter would shag sheep." Goudy's statement inspired the title of the book Stop Stealing Sheep, an introduction to typography.
Word processing and desktop publishing programs for personal computers, such as Microsoft Word, Microsoft Publisher, WordPerfect, QuarkXPress, Adobe InDesign, Adobe FrameMaker, Adobe Illustrator, and Adobe Photoshop, use differing methods of adjusting letter-spacing. Most systems have the default letter-spacing at zero and instead use the character widths and kerning information built into the font itself.
Although digital type sets tighter than metal type on average, this results primarily from the availability of kerning. Digital type does allow for negative sidebearings, which were uncommon in metal type because of the difficulty in cutting a "kern".
In the days of machine-implemented lead typesetting, such as Linotype machines and the Monotype System, letter-spacing had to be uniform. In modern 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 an em, and Adobe InDesign uses 1/1000 of an em. Therefore, in QuarkXPress, a tracking setting of 3 reduces the visual density of the text noticeably, but in InDesign a tracking setting of 3 is barely noticeable.
The amount of letter-spacing in text affects legibility. Tight letter-spacing, especially in small text sizes, can diminish legibility. Adding whitespace around the characters allows the individual characters to emerge and to be recognized more quickly. Adding too much space, however, may isolate individual letters and make it harder for the reader to perceive whole words and phrases, reducing readability.
Letter-spacing may also refer to the insertion of fixed spaces, as was commonly done in hand-set metal type to achieve letter-spacing. Fixed spaces vary by size and include hair spaces, thin spaces, wordspaces, en-spaces, and em-spaces. An en-space is equal to half the current point size, and an em-space is the same width as the current point size.
With CSS1, a standard of 1996, the letter-spacing property (illustrated) offers some control for "kerning perception", as kerning can be simulated with non-uniform spacing between letters. The CSS3 standard includes the font-kerning property. In the meantime, web designers used the workaround of letter-spacing, mainly to enhance spaced texts of titles and banners.
The dictionary definition of letterspacing at Wiktionary