The language is described in the book The Joy of X. is a macro package, not a modification to the source program, so all the capabilities described in tex(1) are present.
The macros encourage writers to think about the content of their documents, rather than the form. The ideal, not always realized, is to have no formatting commands (like ``switch to italic'' or ``skip 2 picas'') in the document at all; instead, everything is done by specific markup instructions: ``emphasize'', ``start a section''.
is the official typesetting system of the American Mathematical Society, and nearly all of its publications are typeset using . The online version of AMS Math Reviews uses input syntax for display of mathematical material. AMS authors can provide editors with computer-readable files.
For authors more familiar with X, an AMS-supported package called amsmath.sty is is available.