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############################################################
Modes
############################################################
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".