Most commonly, it is a crescentic fold of endocardium arising from the anterior rim of the IVC orifice. The lateral horn of the crescent tends to meet the lower end of the crista terminalis, while the medial horn joins the thebesian valve, a semicircular valvular fold at the orifice of the coronary sinus. At the other extreme, it persists as a mobile, elongated structure projecting several centimeters into the right atrial cavity.
Most commonly, it is a curved fold of heart tissue emerging from the front edge of the IVC opening. The outer horn of this curve usually meets the lower end of the terminal ridge, while the inner horn attaches to the thebesian valve, a semicircular flap-like structure at the coronary sinus orifice. At the other extreme, it remains as a flexible, elongated projection extending into the right atrium.