X. The Synoptic Problem

About 90% of Mark's material appears in Matthew. About 50% of Mark appears in Luke. Matthew and Luke share another large body of material not in Mark, mostly sayings of Jesus. They are called the Synoptic Gospels ("seen together"). John, by contrast, is sui generis.

The question: what is the literary relationship?

This is the Two-Source Hypothesis. Not the only theory (the Farrer hypothesis dispenses with Q and has Luke using Matthew), but the consensus.

John shares maybe 8% with the Synoptics. He's writing for a different purpose: not to retell, but to interpret. Why these signs, what they mean. John 20:31 — "these are written that you may believe."