This can be used to stack something above something else; used in math mode.
\stackrel{top}{bot}
The first argument, top, is typset immediately above the
second argument, bot; the former is set in the same
math style as a superscript.
For example
H$_2$CO$_3$ $\stackrel{\rm heat}
{\longrightarrow}$ H$_2$O + CO$_2$
will produce something like
heat
H CO ------> H O + CO
2 3 2 2
Note that the the \rm declaration is used
so that "heat" is set in Roman face and not math italics.
The argument is considered to be a single variable sized math symbol for purposes of placing limits below (subscripts) and above (superscripts) in display math style. For example,
\mathop{\sum \sum}_{i,j=1}^{N} a_i a_j
would produce something like
N
--- ---
\ \
/ / a a
--- --- i j
i,j
It is possible to create two rows of "subscripts" by using
a similar construction: The first row is created as subscripts of the symbol
in the usual way and a lower row is created by placing this whole construct
as the argument of a \mathop command with its own subscript. For
example
\mathop{\sum_{i,j=1}^{N}}_{i>j}
would produce something like
N
---
\
/
---
i,j=1
i>j