  We calculate couplings of arbitrary order from correlation functions among
twisted strings, using conformal field theory. Twisted strings arise in
heterotic string compactified on orbifolds yielding matter fields in the low
energy limit. We calculate completely the classical and the quantum amplitude
including normalization, up to a contribution from Kahler potential. The
classical action has saddle points which are interpreted as worldsheet
instantons described by metastable untwisted strings, formed by twisted strings
distributed at certain fixed points. This understanding generalizes the area
rule, in the case that the locations of twisted strings do not form a polygon,
and provides a general rule for calculating these kinds of instanton
corrections. An interpretation of couplings involving linearly combined states
is given, which commonly appear in non-prime order orbifolds. The quantum part
of the amplitude is given by ratios of gamma functions with order one
arguments.
