AMS-LaTeX

AMS-LaTeX is a collection of LaTeX document classes and packages developed for the American Mathematical Society. Its additions to LaTeX include the typesetting of multi-line and other mathematical statements, document classes, and fonts containing numerous mathematical symbols.
It has largely superseded the plain TeX macro package AMS-TeX. AMS-TeX was originally written by Michael Spivak, and was used by the AMS from 1983 to 1985.
MathJax supports AMS-LaTeX through extensions.
The following code of the LaTeX2e produces the AMS-LaTeX logo:

%%% -- AMS-LaTeX_logo.tex -------
\documentclass
\usepackage
\begin
\AmS-\LaTeX
\end

The package has a suite of facilities to format multi-line equations. For example, the following code,

\begin
y &= ^2 \
&= x^2+2x+1
\end

causes the equals signs in the two lines to be aligned with one another, like this:
AMS-LaTeX also includes many flexible commands for formatting and numbering theorems, lemmas, etc. For example, one may use the environment

\begin Suppose $a\leq b\leq c$ are the side-lengths of a right triangle.\\ Then $a^2+b^2=c^2$.\end
\begin... \end

to generate

Theorem Suppose ''are the side-lengths of a right triangle.
Then''.
Proof... □