Commands to assist the reuse of boxes (set up by \sbox or by the 'lrbox' environment); the '\settobox...' commands behave similarly to the \settowidth (etc.) commands. For example:

      \newsavebox{\mybox}
      \newlength{\mylength}
      \sbox{\mybox}{Hello World}
      \settoboxwidth{\mylength}{\mybox}
    

The author is Heiko Oberdiek.