LaTeX error · Math mode

Double subscript

The error in the log

This is how the message appears in the .log file (and in the compile output). The line starting with l. is the line number where TeX noticed the problem; the real mistake is there or just before it.

! Double subscript.
l.3 The entry $a_i_
                   j$ of the matrix.
I treat `x_1_2' essentially like `x_1{}_2'.

Why it happens

• A multi-character index typed without braces, such as a_i_j for the matrix entry a with indices i and j.
• A nested script (a subscript of a subscript) written as x_n_k instead of x_{n_k}.
• A macro that already adds a subscript, used with another _ after it, for example \newcommand{\xs}{x_i} followed by \xs_j.
• A prime after a superscript, as in f^2': the prime is itself a superscript. Write f'^2, which LaTeX merges into one superscript, or {f^2}'.

How to fix it

Put everything that belongs in one index inside a single pair of braces. a_{ij} typesets both letters as one subscript; a_{i,j} adds a comma, which reads better with longer indices.

If you really want a script on a script, nest it: x_{n_k} puts k below n. To attach a second subscript to an expression that already has one, brace the expression first: {a_i}_j.

Broken: reproduces the error

\documentclass{article}
\begin{document}
The entry $a_i_j$ of the matrix.
\end{document}

Fixed: compiles cleanly

\documentclass{article}
\begin{document}
The entry $a_{ij}$ of the matrix, the sequence $x_{n_k}$
and the power $x^{2^3}$.
\end{document}

Double superscript

! Double superscript.
l.3 $x^2^
         3$

The same rule applies to superscripts: x^2^3 has two ^ on one base. Write x^{2^3} for a tower of powers or {x^2}^3 for (x²)³, which most readers would rather see as (x^2)^3.

A prime counts as a superscript, so f^2' triggers it too. Put the prime first, f'^2, which LaTeX combines into a single superscript, or brace the power: {f^2}'.

Related errors

• Missing $ inserted – A math-only symbol (_, ^, \alpha) was used in text, or math mode was left early.
• Missing } inserted – A brace group was still open when math mode or a table cell ended.
• Extra }, or forgotten $ – A } with no matching {, or a group that crosses a $ boundary.
• All LaTeX errors and warnings – The full list, grouped by cause

Frequently Asked Questions

How do I write a subscript with two letters in LaTeX?

Put both letters in braces: a_{ij}. Without braces only the first character after the underscore becomes the subscript, and a second underscore causes the "Double subscript" error.

How do I write a subscript of a subscript?

Nest it inside braces: x_{n_k} puts k as a subscript of n, which is itself the subscript of x. The inner script is set in an even smaller size automatically.

Why do I get "Double superscript" with a prime?

In math mode a prime is a superscript, so f^2' puts two superscripts on f. Write the prime first, f'^2, which LaTeX merges into one superscript, or brace the power: {f^2}'.