(latex2e.info)Modes

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17 Modes
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As LaTeX processes your document, at any point it is in one of six
modes.  They fall into three categories of two each, the horizontal
modes, the math modes, and the vertical modes.  Some commands only work
in one mode or another (in particular, many commands only work in one of
the math modes), and error messages will refer to these.

   * "Paragraph mode" is what LaTeX is in when processing ordinary text.
     It breaks the input text into lines and breaks the lines into
     pages.  This is the mode LaTeX is in most of the time.

     "LR mode" (for left-to-right mode; in plain TeX this is called
     "restricted horizontal mode") is in effect when LaTeX starts making
     a box with an '\mbox' command.  As in paragraph mode, LaTeX's
     output is a string of words with spaces between them.  Unlike in
     paragraph mode, in LR mode LaTeX never starts a new line, it just
     keeps going from left to right.  (Although LaTeX will not complain
     that the LR box is too long, when it is finished and next tries to
     put that box into a line, it could very well complain that the
     finished LR box won't fit there.)

   * "Math mode" is when LaTeX is generating an inline mathematical
     formula.

     "Display math mode" is when LaTeX is generating a displayed
     mathematical formula.  (Displayed formulas differ somewhat from
     inline ones.  One example is that the placement of the subscript on
     '\int' differs in the two situations.)

   * "Vertical mode" is when LaTeX is building the list of lines and
     other material making the output page.  This is the mode LaTeX is
     in when it starts a document.

     "Internal vertical mode" is in effect when LaTeX starts making a
     '\vbox'.  This is the vertical analogue of LR mode.

For instance, if you begin a LaTeX article with 'Let \( x \) be ...'
then these are the modes: first LaTeX starts every document in vertical
mode, then it reads the 'L' and switches to paragraph mode, then the
next switch happens at the '\(' where LaTeX changes to math mode, and
then when it leaves the formula it pops back to paragraph mode.

   Paragraph mode has two subcases.  If you use a '\parbox' command or a
'minipage' then LaTeX is put into paragraph mode.  But it will not put a
page break here.  Inside one of these boxes, called a "parbox", LaTeX is
in "inner paragraph mode".  Its more usual situation, where it can put
page breaks, is "outer paragraph mode" (Note: Page breaking).

\ensuremath

Ensure that math mode is active.