The scholarly question of how the three Synoptic Gospels relate to each other. Perhaps the most common approach to the relation of the three Synoptic Gospels is the “two source” theory, which claims that Matthew and Luke used Mark as a source, while also drawing upon a second source (usually known as “Q”). Other possibilities exist: for example, the Grisebach hypothesis, which treats Matthew as having been written first, followed by Luke and then Mark.