To swear an oath by Jerusalem is to invoke God, seeing as he has chosen the city of Jerusalem as the place for his temple (Psalm 48:2; Matthew 4:5; Matthew 27:53). God is witness to your oath when you swear by Jerusalem, it is not a lesser oath that you are not obligated to keep.
35 or by the earth, for it is his footstool, or by Jerusalem, for it is the city of the great King.