This chart outlines the so-called "Four Sources Hypothesis" (sometimes called the "Two-Source Hypothesis") regarding the hypothetical sources for the three synoptic Gospels (Matthew, Mark, and Luke).

The Gospels are called synoptic because they follow the same outline and structure, and can be read side-by-side ("syn-opsis" = "in common view"). Sometimes the similarities are word-for-word.

The hypothesis states that four, original sources circulated some time in the first century:

Q: a hypothetical "sayings source" (i.e., with almost no narrative and most sayings attributed to Jesus). "Q" comes from the German word Quelle (pronounced: Kuh-vell-uh), which just means "source"). We have no copy of "Q": it has been deduced from the sayings that both Matthew and Luke have, but that Mark does not

Mark: a narrative source: either the Gospel of Mark itself, or some other, very similar narrative source that is very, very close to the Gospel of Mark that we possess.

M: a hypothetical source of narratives and sayings used by the author of the Gospel of Matthew

L: a hypothetical source of narratives and sayings used by the author of the Gospel of Luke