UK TeX FAQ -- question label nodollar

### "Missing `$` inserted"

There are certain things that only work in maths mode. If your document is not in maths mode and you have an `_` or a `^` character, TeX (and by inheritance, LaTeX too) will say

    ! Missing $ inserted

as if you couldn't possibly have misunderstood the import of what you were typing, and the only possible interpretation is that you had commited a typo in failing to enter maths mode. TeX, therefore, tries to patch things up by inserting the `$` you 'forgot', so that the maths-only object will work; as often as not this will land you in further confusion.

It's not just the single-character maths sub- and superscript operators: anything that's built in or declared as a maths operation, from the simplest lower-case `\``alpha` through the inscrutable `\``mathchoice` primitive, and beyond, will provoke the error if misused in text mode.

LaTeX offers a command `\``ensuremath`, which will put you in maths mode for the execution of its argument, if necessary: so if you want an `\``alpha` in your running text, say `\``ensuremath{`\``alpha`}`; if the bit of running text somehow transmutes into a bit of mathematics, the `\``ensuremath` will become a no-op, so it's pretty much always safe.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=nodollar