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Modes
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When LaTeX is processing your input text, it is always in one
of three modes:
Paragraph mode
Math mode
Left-to-right mode, called LR mode for short
LaTeX changes mode only when it goes up or down a staircase
to a different level, though not all level changes produce
mode changes. Mode changes occur only when entering or
leaving an environment, or when LaTeX is processing the
argument of certain text-producing commands.
"Paragraph mode" is the most common; it's the one LaTeX is
in when processing ordinary text. In that mode, LaTeX breaks
your text into lines and breaks the lines into pages. LaTeX
is in "math mode" when it's generating a mathematical formula.
In "LR mode", as in paragraph mode, LaTeX considers the output
that it produces to be a string of words with spaces between
them. However, unlike paragraph mode, LaTeX keeps going from
left to right; it never starts a new line in LR mode. Even if
you put a hundred words into an \mbox, LaTeX would keep
typesetting them from left to right inside a single box, and
then complain because the resulting box was too wide to fit
on the line.
LaTeX is in LR mode when it starts making a box with an \mbox
command. You can get it to enter a different mode inside the
box - for example, you can make it enter math mode to put a
formula in the box. There are also several text-producing
commands and environments for making a box that put LaTeX in
paragraph mode. The box make by one of these commands or
environments will be called a parbox. When LaTeX is in paragraph
mode while making a box, it is said to be in "inner paragraph
mode". Its normal paragraph mode, which it starts out in, is
called "outer paragraph mode".
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