There's also a command called sideset, for a rather special
purpose: putting symbols at the subscript and
superscript corners of a
large operator symbol such as 
or 
. The prime example is
the case when you want to put a prime on a sum symbol. If there are no
limits above or below the sum, you could just use nolimits: here's
|' E_n| in display mode:
If, however, you want not only the prime but also something below or above the sum symbol, it's not so easy---indeed, without sideset, it would be downright difficult. With sideset, you can write
\sideset{}{'}\sum_{n<k,\;\text{$n$ odd}} nE_n
to get
The extra pair of empty braces is explained by the fact that sideset has the capability of putting an extra symbol or symbols at each corner of a large operator; to put an asterisk at each corner of a product symbol, you would type
\sideset{_*^*}{_*^*}\prod
producing
