When reading input TeX converts the stream of read characters into a
sequence of tokens. When TeX sees a backslash \, it will
handle the following characters in a special way in order to make a
control sequence token.
The control sequences fall into two categories:
\ followed by at least one ASCII letter (A-Z and
a-z), followed by at least one non-letter.
\ followed by one non-letter character.
The sequence of characters so found after the \ is also called
the control sequence name.
Blanks after a control word are ignored and do not produce any
whitespace in the output (see \newcommand & \renewcommand and
Backslash-space, \ ).
Just as the \relax command does nothing, the following input
will simply print ‘Hello!’ :
Hel\relax lo!
This is because blanks after \relax, including the newline, are
ignored, and blanks at the beginning of a line are also ignored
(see Leading blanks).