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Mitral annular disjunction (MAD) is an abnormal insertion of the hinge line of the posterior mitral leaflet on the atrial wall: the mitral annulus shows a separation or “disjunction” between the leaflet–atrial wall junction and the crest of the left ventricle myocardium. This anomaly is often observed in patients with myxomatous mitral valve prolapse. The anatomical substrate of MAD remains unclear for the following reasons: (1) most studies are focused on the association between MAD and arrhythmias, rather than on pathomorphological aspects of MAD; and (2) the complex anatomic architecture of the posterior mitral annulus is often simply described as the posterior segment of a fibrous ring. The aims of this paper are to review the pertinent normal anatomy of the mitral valve and to propose new hypotheses on the morphological nature of MAD.