\stackrel

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.

\mathop

This command does not seem to be well documented but appears to work, at least in current versions of LaTeX 2.09.

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