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The Q document or Q (from the German Quelle, "source") is a postulated lost textual source for the Gospel of Matthew and the Gospel of Luke.
In the nineteenth century, New Testament scholars noted that the gospels of Matthew and Luke shared much material in common, which was echoed in the Gospel of Mark. They suggested a second common source, which was termed the Q document. This hypothetical lost text—also called the Q Gospel, the Sayings Gospel Q, the Synoptic Sayings Source, the Q Manuscript, and (in the nineteenth century) The Logia—seems most likely to have comprised of a collection of Jesus' sayings. Recognizing such a Q document is one of two key elements in the "two-source hypothesis" alongside the priority of Mark.
The two-source hypothesis is the most widely accepted solution to the so-called "Synoptic Problem," which concerns the literary relationships among the first three canonical gospels (the Gospels of Mark, Matthew, and Luke), known as the Synoptic Gospels. Similarity in word choices and event placement shows an interrelationship. The synoptic problem concerns how this interrelation came to pass and what the nature of this interrelationship is. According to the two-source hypothesis, Matthew and Luke both used the Gospel of Mark, independently of one another. This necessitates the existence of a hypothetical source in order to explain the double tradition material where there is agreement between Matthew and Luke that is not in Mark. This hypothetical source is named Q for convenience.
The synoptic problem
Of the four canonical gospels included in the New Testament, Matthew, Mark, and Luke—due to common narratives and viewpoints—are called the Synoptic Gospels (from Greek synoptic, meaning "seeing with the same eyes").
Their similarities exceed mere congruency in perspective, and are not easily attributed to coincidence. The narrative is retold in a common sequence, and nearly identical phrases appear in all of the works. It has been long proposed by serious reviewers that the books were based on a common published source or sources, given the improbability of three eyewitnesses producing verbatim accounts of their experiences.
The recognition of this synoptic problem, and attempts to resolve it, date to antiquity. The fifth-century bishop Augustine of Hippo posited that Matthew was written first, then Mark was written using Matthew as a source, and finally Luke was written using Matthew and Mark as sources. This early and influential explanation is no longer supported by modern scholars.
https://www.newworldencyclopedia.org/entry/Q_Document