Allocate a new length register (see Lengths). The required argument
\len has to be a control sequence (see Control sequence, control word and control symbol), and as such must begin with a backslash,
normal circumstances. The new register holds rubber lengths such as
1in plus.2in minus.1in (a LaTeX length
register is what plain TeX calls a
skip register). The
initial value is zero. The control sequence
\len must not
be already defined.
If you forget the backslash then you get ‘Missing control sequence inserted’. If the control sequence already exists then you get something like ‘LaTeX Error: Command \graphichgt already defined. Or name \end... illegal, see p.192 of the manual’.