A harmonic punctuation mark. Authentic (5 → 1) is the period, plagal (4 → 1) the amen, half (stop on 5) the comma, deceptive (5 → 6-) the plot twist. Rock leans so hard on plagal motion that 4 → 1 does real closing work there. Every device on this site is a scheme for approaching, delaying, or subverting one of these.