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 2Note 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_jwould produce something like
N --- --- \ \ / / a a --- --- i j i,jIt 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