  We deal with the monadic (second-order) theory of order. We prove all known
results in a unified way, show a general way of reduction, prove more results
and show the limitation on extending them. We prove (CH) that the monadic
theory of the real order is undecidable. Our methods are model-theoretic, and
we do not use automaton theory.
  This is a slightly corrected version of a very old work.
