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I'm new to TeX and tried to enter an equation using \mathbb. I received a prompt

LaTeX Error: \mathbb allowed only in math mode.
See the LaTeX manual or LaTeX Companion for explanation.

I thought I would try here for an explanation.

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    Phil's answer below is correct, but a further tip: the macro \mathbb typesets capital letters as "black-board bold" symbols, such as the symbol ℝ which usually denotes the set of real numbers (and which is produced by \mathbb{R} in math mode). – Niel de Beaudrap Apr 14 '11 at 6:48
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You enter math mode by typing a dollar sign $, and you leave math mode by typing another dollar sign $. You should take a look at Chapter 3 of The Not So Short Introduction to LaTeX

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TeX knows several modes, two of them are math mode and display math mode. Typesetting of ordinary text such as words nor numbers is different compared to mathematical formulas or a mathematical identifier such as x, which should look different from the text x (it does differ here in this editor only vaguely)

In order to let TeX know which mode should be used, there are indicators to enter math mode:

  • Inline math mode, being entered $ and closed with $
  • Display math mode, used with an enclosing $$...$$ in plain TeX or with \[...\] in LaTeX
  • Typing inside \begin{math}...\end{math} in LaTeX
  • Several math environments in LaTeX enter the math mode automatically, such as equation, align, alignedat, gather etc.

Several characters/symbols being connected to math content require entering the math mode, such as \mathbb{R} etc.

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I guess that an even better way to enter math mode is to use \( and \), as discussed in this answer.

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