" Complex sentences are traditionally divided into two basic types: (i) sentences including coordinate clauses , and (ii) sentences including subordinate clauses . The former consist of two (or more) clauses that are functionally equivalent and symmetrical, whereas the latter consist of two (or more) clauses that constitute an asymmetrical relationship: a subordinate clause and a matrix clause do not have equal status and equal function (cf. Foley and Van Valin 1984: 239)... I suggest that prototypical subordinate clauses carry the following features: they are (i) syntactically embedded, (ii) formally marked as a dependent clause, (iii) semantically integrated in a superordinate clause, and (iv) part of the same processing and planning unit as the associated matrix clause."
(Holger Diessel, The Acquisition of Complex Sentences . Cambridge University Press, 2004)