# Documentation of \mathop

The Dirac delta distribution is ubiquitous in physics. Since it is a special object I wanted to give it some extra spacing by using \mathop. I know that single letters inside \mathop are centred around the math axis, whereas multiple letters are aligned with the baseline. Here is a simple Plain TeX example:

\setbox0=\hbox{$\mathop{\delta} \mathop{\delta{}}$}
\noindent\rlap{\vrule width \wd0 height .1pt}\box0
\bye

However, I was not able to find this behaviour documented in the TeXbook. Can someone suggest a reference for this? I really would like to know the exact rules of when a \mathop is centred and when it is aligned at the baseline.

Shift box~$x$ down by ${1\over2}\bigl(h(x)-d(x)\bigr) -a$, where $a=\sigma_{22}$, so that the operator character is centered vertically on the axis; this shifted box becomes the nucleus of the Op atom.