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Control sequence, control word and control symbol

When reading input TeX converts sequences 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:

• control word, when the control sequence is gathered from a \ followed by at least one ASCII letter (A-Z and a-z), followed by at least one non-letter.
• control symbol, when the control sequence is gathered from a \ 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).