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


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


compiles while


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

  • 1
    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
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    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


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

Some more words

The LaTeX manual always uses


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


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


will fail with

! Missing { inserted.
<to be read again> 
l.3 $A_\in+A_\notin

because \notin is not really a single object.

An input such as A_\tilde{x} will similarly fail. On the other hand,


will work, but it's just luck. So, what's the rule?

Sorry, there's none except always using braces.

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