From an anatomical perspective, the term “mitral annulus” is a misnomer. The essential structure supporting the valvular leaflets is the left atrioventricular junction, this being the D-shaped orifice formed at the confluence between the LA walls and the supporting LV structures (2,10,17- 18). On the ventricular aspect, these supporting structures are not exclusively myocardial, because there is an extensive area of fibrous continuity (the so-called aortic-mitral curtain) between the anterior leaflet of the MV and the aortic valve in the roof of the LV (Figure 1). When considered 3-dimensionally, the overall atrioventricular junction is nonplanar, with elevated septal and lateral segments at the ends of the solitary zone of apposition between the leaflets and complementary depressed medial segments along the central component of the zone of apposition, giving a characteristic saddle-shaped overall appearance (Figure 2) (19). It is along the depressed anterior segment of the junction that 1 of the leaflets of the valve is in fibrous continuity with the noncoronary and left coronary leaflets of the aortic valve, this extensive area being well described as the aortic-mitral curtain (Figure 1). At either end, the central part of the fibrous curtain, which represents the annulus of the aortic or anterior leaflet of the valve, is attached by fibrous expansions, the left and right fibrous trigones, to the ventricular myocardium ((Figure 3) and Figure 4). The right trigone is itself continuous with the membranous septum, the combined entity forming the central fibrous body, and with the penetrating part of the atrioventricular conduction axis passing through the atrioventricular component of the membranous septum (18). Fibroelastic cords of various firmness and structure extend from the fibrous trigones through the mural part of the left atrioventricular junction. It is rare for the cords to form a complete ring to support the mural, or posterior, leaflet of the valve. The so-called annulus, therefore, is much more resistant to pathologic dilation along the aortic as opposed to the mural leaflet. Especially in its posterior aspects, several deficiencies in the annular structure are filled with adipose tissue. The absence of a well-formed fibrous cord in this particular position opposite the aortic-mitral curtain explains its predilection for annular dilation and calcification, which result in a disproportional increase in the aortic-to-mural, or septal-to-lateral, diameter of the valvular orifice, potentially precluding adequate leaflet coaptation.