I've just rendered the following:

```
\documentclass[varwidth=true, border=2pt]{standalone}
\begin{document}$\textbackslash$
\end{document}
```

which seems to give the same as `$n$`

. Do you know why?

As `\textbackslash`

is for textmode and not for mathmode, I've expected TeX to fail in this situation. Instead, I get

LaTeX Warning: Command \textbackslash invalid in math mode

So why does `\textasciitilde`

in math mode make LaTeX crash but `\textbackslash`

not?

## Best Answer

The definition of

`\textbackslash`

when the OT1 encoding is used isBoth the first and the third token can't be typed by a user without some devious trick. However, the definition of

`\OMS-cmd`

is equivalent toThe macro

`\@inmathwarn`

is responsible for the warningAfter that the macro

`\OMS\textbackslash`

is expanded. If we were in text mode, the command would do a font change in a group, selecting a font in the OMS encoding (math symbols), doingand a backslash would appear, because that's what an OMS encoded font has at slot

`"6E`

(hexadecimal).If we're in math mode, the same

`\char"6E`

instruction would be performed, but here the font change has no effect. So we get the character at position`"6E`

in the font in math family 0, where an`n`

is found. When`\char"6E`

is found in math mode, TeX does as if it were`\mathchar"006E`

and in family 0, slot`"6E`

there's an`n`

(it's the upright text font and the ASCII code of`n`

is exactly`"6E`

).Things are different in the T1 encoding, because in this case the definition of

`\textbackslash`

isand the latter macro eventually expands to

`\char"5C`

. In a text font, T1 encoding, at slot`"5C`

there's actually a backslash.Here's the explanation. Now, some advice:

Never underestimate warnings and

never use

`\text...`

symbol commands in math mode (don't mistake them with`\textrm`

,`\textit`

or similar, which are obviously legal in math mode).