In the non-printing characters documentation, I see two named characters that are largely undocumented: \[InvisiblePrefixScriptBase] and \[InvisiblePostfixScriptBase].
Interestingly, they have input aliases defined: [Esc]i-[Esc] and [Esc]-i[Esc].
What are they used for?

TraditionalForm[Hypergeometric2F1[a, b, c, d]], and (2) this Q/A where it is used to typset $_{n}C_{r}$ – kguler Dec 6 '12 at 7:26\[InvisiblePostfixScriptBase]-- who knows? Maybe for right-to-left scripts it works similar to the Prefix one? – kguler Dec 6 '12 at 11:32\[InvisiblePostfixScriptBase]to emulate the spaces and alignments in some tensor notions. In LaTeX it's usually done by\phantomcommand. CompareRowBox[{SubscriptBox["F","a"],SubsuperscriptBox["\[InvisiblePostfixScriptBase]","b","c"]}]//DisplayFormto LaTeX codeF_{ab}^{\phantom{a}c}. – Silvia Dec 6 '12 at 12:07