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