Psalm 138 is the first of 8 songs written by David which appear as a set in the closing chapters of the Psalter. These Davidic psalms are followed by the final collection of S songs which all begin and end with the refrain: "Praise the lord!". These closing medleys swerve to reprise two central themes of the entire book - prayer, and praise. David's songs are structured according to the former followed by the crescendo psalms emphasizing the latter. It seems appropriate that this greatest of song books would culminate with a universal call to worship and 8 songs penned as examples by Israel's greatest king.