No one knows for sure what theme was followed in arranging the five books. They seem to have been compiled somewhat independently and then brought together into one collection at a later date. There is some duplication: Psalm 14 in Book 1 is repeated as Psalm 53 in Book 2; a portion of Psalm 40 in Book 1 is repeated as Psalm 70 in Book 2; and the latter halves of Psalms 57 and 60 in Book 2 are combined as Psalm 108 in Book 5.