When reading input TeX converts the sequences of read characters into
a sequence of tokens. When TeX sees a backslash
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, followed by at least one blank. The sequence of at least one ASCII letter is called the control sequence name.
\followed by one non-letter character.
Just as the
\relax command does nothing, the following will print
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).