It is widely supposed that Luke used Mark's gospel and other material as sources. According to this theory some of this other material, often designated “Q” (from Quelle, German for “source”), was also used by Matthew. Some of it, sometimes designated “L” (for “Luke”), was used only by Luke. Another source may have provided the birth, infancy, and boyhood stories. It is even suggested that there may have been an earlier edition of Luke's gospel combining the material designated as “Q” and “L.”
There are other views of the relationship between the first three gospels. The traditional theory is that Matthew was written first, then Mark, and that Luke had access to both of them. Some scholars have suggested that Luke made use of Matthew but was written before Mark. Yet others have claimed that the first three gospels were written independently of each other. But the theory that regards Mark as the earliest of the gospels continues to have the greatest acceptance.