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Parallel accounts
Two books tell the same event or saying. Reading them side by side fills in details one writer leaves out.
Matthew 28:19from
Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
And he said unto them, Go ye into all the world, and preach the gospel to every creature. He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
How to spot it: Look for the same scene told by another writer: the four Gospels, or Kings and Chronicles.