> For the complete documentation index, see [llms.txt](https://overleaf-pro.ayaka.space/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://overleaf-pro.ayaka.space/latex/latex-errors/04-double-subscript.md).

# Double subscript

## Introduction

By convention, LaTeX uses the underscore character (`_`) to typeset a subscript, such as `$a_b$` which produces $$a\_b$$. If you write `$a_bcde$` only the first character, `b`, is subscript, $$a\_bcde$$, because LaTeX does not *assume* you want to subscript the entire set of characters `bcde`. To typeset multiple items (tokens) as subscripts, you need to tell LaTeX by enclosing them in a group `{...}`, like this: `$a_{bcde}$`, which produces $$a\_{bcde}$$.

## Double subscript error

The double subscript error arises when LaTeX is asked to add a subscript to a piece of mathematics which *already has* a subscript attached to it. This error is usually resolved through the use of braces `{...}` which, within math mode, generate a so-called *subformula*—a term used to describe a fragment of the mathematical expression you are trying to typeset.

For example, writing `$a_b_c$` generates the following double subscript error:

![A double subscript error showing on Overleaf](/files/qtM1KaCUMcytjN6lgOOI)

This particular error can be fixed in several ways through the use of braces `{...}`—the results vary according to where the braces are positioned:

* `$a_{b_c}$` typesets $$\displaystyle a\_{b\_c}$$
* `${a_b}_c$` typesets $${a\_b}\_c$$
* `$a_{bc}$` typesets $$a\_{bc}$$

### Double subscript error: a special case

A question on `tex.stackexchange` highlights [a surprising example of the double subscript error](https://tex.stackexchange.com/questions/253080/why-am-i-getting-a-double-subscript-error), one whose cause and corresponding fix may be worth knowing about. We reproduce the example here.

The following example fails to compile, even though braces are used:

```latex
\documentclass{article}
\begin{document}
\({\vec a_b}_c\)
\end{document}
```

[Open this ***error-generating*** example in Overleaf](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=accents+package+example\&snip=%5Cdocumentclass%7Barticle%7D%0A%5Cbegin%7Bdocument%7D%0A%5C%28%7B%5Cvec+a_b%7D_c%5C%29%0A%5Cend%7Bdocument%7D)

One way to fix the problem is to add the [`accents` package](https://ctan.org/pkg/accents?lang=en) to your document preamble (as suggested [here](https://tex.stackexchange.com/a/253094)):

```latex
\documentclass{article}
\usepackage{accents}
\begin{document}
\({\vec a_b}_c\)
\end{document}
```

[Open this example in Overleaf](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=accents+package+example\&snip=%5Cdocumentclass%7Barticle%7D%0A%5Cusepackage%7Baccents%7D%0A%5Cbegin%7Bdocument%7D%0A%5C%28%7B%5Cvec+a_b%7D_c%5C%29%0A%5Cend%7Bdocument%7D)

## More on subscripts

Further levels of subscripts can be typeset by writing LaTeX such as `$a_{b_{c_{d_e}}}$` which typesets $$a\_{b\_{c\_{d\_e}}}$$. All subscripts at or below the second level (sub-subscript) are typeset using the same font size (in points), whereas first-level subscripts are typeset using a slightly larger font (point size). To learn more about subscripts, visit Overleaf's [help page dedicated to that topic](/latex/mathematics/02-subscripts-and-superscripts.md).


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