(a) The Synoptic Gospels are so similar in order and wording that there must be some kind of literary link between them.
(b) Mark is consistently the “middle term”, i.e. agreements between Matthew and Luke are often “mediated” via Mark. Matthew and Luke are rarely the middle term.
(c) The easiest way to explain Mark as middle term is that it was the major source for both Matthew and Luke. This explains all the “triple tradition” (Matthew // Mark // Luke) material.
(d) But two hundred verses or so of major agreements between Matthew and Luke remain, the “double tradition” (Matthew // Luke).
(e) If Matthew and Luke used Mark independently, the double tradition must be derived from an hypothetical source, called “Q” (for Quelle, source).
(f) Thus the “Two Source Theory”, that Matthew and Luke had two major sources, Mark and Q. It is the majority view.
(g) But there is good evidence that Luke knew Matthew as well as Mark, e.g. some major and many minor agreements between Matthew and Luke against Mark.
(h) Q is therefore unnecessary. Matthew knew Mark; Luke knew Mark and also Matthew. This is called the Farrer Theory.