Phonocardiograms in three frequency bands confirm resemblance of systolic clicks due to mitral cusp prolapse to opening snap of mitral stenosis. Both sounds are brief and higher pitched than first, second, or third heart sounds. A similar sound was evoked by tensing anterior mitral leaflets from commissure to commissure, parallel to the annulus, while tensing from the papillary muscles to the annulus evoked sounds similar to the first sound in pitch and duration. Lateral tension on the anterior cusp is suddenly applied in patients with systolic prolapse of the mitral valve when the posterior cusp is pulled away. This leaves the anterior cusp without counter-pressure at the line of closure and it must expand to its maximal area under full systolic tension.