Odd:

https://www.catholic.com/magazine/online-edition/some-thoughts-on-q

Q

Definition

The symbol of the hypothetical document used by the authors of the first and third Gospels, along with St. Mark, to produce the present Gospels according to Matthew and Luke. The existence of "Q" is based on the assumption that Mark and not Matthew was the first Gospel and that an unknown document (hence the German Quelle, source) must be supposed to account for similarities in Matthew and Luke that are not found in Mark. There are no grounds in historical tradition for the existence of such a source.

https://www.catholicculture.org/culture/library/dictionary/index.cfm?id=35928