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PostScript has been for many years a lingua franca of powerful printers (though modern high-quality printers now tend to require some constrained form of Adobe Acrobat, instead); since PostScript is also a powerful graphical programming language, it is commonly used as an output medium for drawing (and other) packages.

However, since PostScript is such a powerful language, some rules need to be imposed, so that the output drawing may be included in a document as a figure without “leaking” (and thereby destroying the surrounding document, or failing to draw at all).

Appendix H of the PostScript Language Reference Manual (second and subsequent editions), specifies a set of rules for PostScript to be used as figures in this way. The important features are:

• certain “structured comments” are required; important ones are the identification of the file type, and information about the “bounding box” of the figure (i.e., the minimum rectangle enclosing it);
• some commands are forbidden — for example, a showpage command will cause the image to disappear, in most TeX-output environments; and
• “preview information” is permitted, for the benefit of things such as word processors that don’t have the ability to draw PostScript in their own right — this preview information may be in any one of a number of system-specific formats, and any viewing program may choose to ignore it.

A PostScript figure that conforms to these rules is said to be in “Encapsulated PostScript” (EPS) format. Most (La)TeX packages for including PostScript are structured to use Encapsulated PostScript; which of course leads to much hilarity as exasperated (La)TeX users struggle to cope with the output of drawing software whose authors don’t know the rules.