To avoid uttering God's name, oaths in Jesus' day were commonly sworn by substitutes: by heaven, by earth, by Jerusalem, by one's own head. Around the substitutes grew a case law of which formulas actually bind. The Mishnah's oath tractate preserves exactly this reasoning: an oath administered “in the name of heaven and in the name of earth” leaves the swearer exempt, because that is not an oath in God's name (Shevuot 4:13). The practical effect was a graded honesty in which some sworn words could be safely broken.

That loophole culture, not courtroom oath-taking as such, is Jesus' first target here, and again in Matthew 23:16-22 where the Temple-versus-gold and altar-versus-gift distinctions are the same casuistry. His answer closes every gap at once: heaven is God's throne, earth His footstool, Jerusalem His city, so every substitute reaches God anyway. A person whose yes is yes needs no formula at all. (Whether this also forbids civil oaths has been debated among Christians, notably by Quakers and Anabaptists; the scene's first edge is the loophole system.)