Rayleigh's principle, based on a circular jet, yields the cell length (defined by the first mode) of a square jet only 4% too long [J. Acoust. Soc. Am. Suppl. 187, S80 (1990)]. Yet the cellstructureis fundamentally different. The square one is dominated by expansive plane waves moving in (at propagation velocityc) from each side (lengthl) reflecting repeatedly from the opposite sides, with axial period corresponding to 2l/c, accompanied by expanding compressive waves centered on each corner that bring the plane waves to ambient pressure at the edges. The axial pressure distribution is quite nonperiodic: The periodic cell length, corresponding to (1/√)×2l/c, is not very overt. In the circular case the converging cylindrical wave “reflects” from the center with a rather impulsive compressive wavefront, resulting in almost ambient pressure across the jet at timeD/c; the original overpressure is almost exactly reversed at time 2D/c. The periodic cell length, however, corresponds to(π/2j0.1) × (2D/c) = 0.6531 × 2D/c. [Work partially supported by the Texas Advanced Research Program.]