X-ACTO KNIFE A cutting knife familiar to model airplane builders and other hobbyists that is used in the printing and graphics arts industries to make precise cuts in artwork and typeset copy, page layouts, etc., when preparing or revising paste-ups. See also LAYOUT; MECHANICAL; PASTE-UP

XEROGRAPHY Electrophotography. See ELECTROPHOTOGRAPHY

XEROX CORPORATION A large U.S. corporation that first gained national prominence as a result of its xerographic, plain-paper copiers. The company, which has headquarters in Stamford, Conn., was founded in 1906 as the Haloid Company. It underwent a name change in 1958 to the Haloid Xerox Company and again in 1961 to the Xerox Corporation. Xerox was the first company to market the xerographic copier; which produces photographic copies on plain, uncoated paper. Today the company manufactures computer terminals, disk drives, software; office supplies; and optical equipment; and conducts research in military and aerospace technology. It has expanded into information products and publishing, and offers seminars and management consulting services. See also PHOTOCOPY

X-HEIGHT In typography, x-height is the vertical height of the lowercase letter x in any specified font. The x-height is also called the body height, as it represents the height of the lowercase letters, excluding ascenders (such as the upper part of the letter b) and descenders (such as the tail on the letter g). The bottom of the x marks the baseline; its top marks the mean line or x-line.

The term x-height derives from the historical use of the letter "x" as the standard for measurement of the lowercase letter size of a typeface. The x-heights of some typefaces are 40% of the height of X, some are 50%, and others are 60%. A typeface having lowercase letters, which are almost as tall as its uppercase letters, is said to have a large x-height. A typeface with a large x-height appears to be larger than a typeface of the same point size that has a small x-height. Consequently, it is easier-to-read. See also ASCENDER; BASELINE; DESCENDER LETTER; LOWERCASE; TYPEFACE

X-LINE The upper edge of x-height. An imaginary horizontal line that borders the top of lowercase letters, such as "x," which have no ascenders. See ASCENDER; LOWERCASE; X-HEIGHT