The name / is used for convenience to describe a set of loosely related files that are distributed together by the American Mathematical Society. Basically they may be described as miscellaneous enhancements to / for superior information structure of mathematical documents and superior printed output. Because / is an extension for /, which in turn is a `macro package'macro package for the / typesetting program, it follows that in order to use any of the pieces of / you need to have / and / installed first.
/ by itself does a rather good job of typesetting mathematics, compared to non-/-based software; it doesn't add much, however, to the basic set of mathematical capabilities that it adopted from the Plain / macro package.
At the same time that / was being developed by Leslie Lamport (roughly 1982--1986), the American Mathematical Society was throwing its resources into the development of a different macro package known as / , written by Michael Spivak . By 1987 or so it became evident that / and / had complementary feature sets: / focused on the typesetting of math formulas and on fine-tuning typically done by publishers, and was relatively weak in other areas (for example no automatic numbering or cross-reference facilities); / focused on document structure and logical markup of text, and had a comparatively limited set of features for dealing with math formula contents. This situation led to dissatisfaction among both / and / users who saw desirable features tantalizingly out of reach in the other macro package. So the American Mathematical Society looked into the question of producing some sort of combination of the two macro packages that would better serve mathematicians in their writing tasks. The decision that was eventually taken was to graft the mathematical capabilities of / onto the base stock of / through an extension package: /. Most of the programming work was done by Frank Mittelbach and Rainer Schöpf in 1989--1990 and version 1.0 of / was released in mid-1990.