I will add that if you use something like this in some post, you might consider using a macro. E.g., if you use $\newcommand{\Map}[2]{{\vphantom{{#2}}}^{#1}{#2}}$ $\newcommand{\Map}[2]{{\vphantom{{#2}}}^{#1}{#2}}$, then in the whole post you can use $\Map AB$ to get $\Map AB$. And if you later want to change the way it looks, you can simply change the macro and it will change all occurrences. (If there are many of them, it is a lot of work to change each one of them.) And to some extent this resolves also the problem that "logically ties the superscript to the character on its right." — Martin Sleziak 49 secs ago