The right coronary artery is "dominant" (contributing to the supply of the left ventricular muscle) in 90% of people. Although the most common pattern is for the right coronary artery to bifurcate at the crux giving the posterior descending (posterior interventricular) artery, a branch may arise before the crux, either as an aberrant acute marginal artery or as an early posterior descending artery, crossing the diaphragmatic surface of the right ventricle. Recognition of this possibility may be important if surgical revascularisation is to be complete. To establish the frequency of these different patterns of blood supply, 22 hearts were studied in the dissecting room, and 100 consecutive coronary angiograms and 100 consecutive operative drawings were reviewed. A vessel arising before the crux contributed to the supply of the left ventricle in one third of cases.