# Why do some commands require explicit braces in subscripts?

Most single commands don't require explicit braces if they appear in a subscript or superscript. For example,

a_\text{n}

compiles fine. But a few do require explicit braces: for example,

a_{\tilde{n}}

compiles while

a_\tilde{n}

does not. Is there any rule for which commands must be surrounded by explicit braces in a subscript even when they appear by themselves?

• The \text macro happens to be defined in a particularly clever way. As a side-effect, as you've discovered, it can appear without curly braces as the argument of a _ subscript initiator. For most other macros, it's necessary to help TeX parse the material correctly by enclosing the macro and the argument(s) in a pair of curly braces. – Mico Mar 7 '16 at 20:56
• in addition to what Mico says, unless the sub/superscript is a single letter or digit, it's never a bad idea to enclose it in braces. – barbara beeton Mar 7 '16 at 21:04

# TL;DR

Always use braces. Always use braces. Always use braces.

# Some more words

The LaTeX manual always uses

_{<subscript>}

even if the subscript consists of a single token and it has very good reasons for doing so; for instance, it would propose

a_{1}+b_{11}

Experienced users tend to omit the braces, but this can be very dangerous. For example, the following example

\documentclass{article}
\begin{document}
$A_\in+A_\notin$
\end{document}

will fail with

! Missing { inserted.
l.3 $A_\in+A_\notin$