The box created by \vphantom has the height and depth of its argument, but its width is zero (so it doesn't contribute to any horizontal spacing issues). In other words, \hphantom creates vertical space equal to that produced by its argument, but doesn't create any horizontal space.

EXAMPLE

```
\binom{\frac ab}c \binom{\vphantom{\frac ab}?}c
```
$ \binom{\frac ab}c \binom{\vphantom{\frac ab}?}c $