[TA] three-sided zone of right atrium bounded by the base of the septal leaflet of the tricuspid valve, the anteromedial margin of the coronary sinus orifice, and the palpable subendocardial collagen bundle (tendon of Todaro) coursing from the right fibrous trigone of the fibrous skeleton of the heart to the valve of the inferior vena cava. The atrioventricular node lies within the interatrial septum in this zone. SYN: triangle of Koch.