Mode-locking regions in the parameter plane of the standard circle map θn+1 = θn + Ω − (K/2π) sin(2πθn). Each tongue is a region where the winding number locks to a rational value p/q. At K = 0 the tongues are infinitely thin (only rational Ω); they widen as coupling K increases and begin to overlap past K = 1, where the map loses invertibility and chaotic dynamics emerge.