This still picture from a left coronary angiogram in right anterior oblique view with a caudal tilt shows the main branches of the left coronary artery. A temporary pacing electrode is also seen, which was inserted as the person was hemodynamically unstable with acute inferior wall myocardial infarction and complete heart block. LMCA: left main coronary artery; LCX: left circumflex coronary artery; LAD: left anterior descending coronary artery. Left main coronary artery divides into left anterior descending coronary artery and left circumflex coronary artery. Diagonals and septals are branches of left anterior descending coronary artery. Obtuse marginal is a branch of the left circumflex coronary artery. In this case the LCX continues as the major obtuse marginal artery (OM) and the parent LCX continuing in the atrioventricular groove is a small vessel which does not reach upto the crux. Crux is the junction of the atrioventricular and interventricular grooves on the posterior surface of the left ventricle. The coronary which crosses the crux to the opposite side is called the dominant artery. In this case left coronary artery is non dominant. The right coronary artery should be dominant and giving rise to the posterior descending coronary artery and posterior left ventricular branches. In co-dominant systems, posterior descending coronary artery originates from one side and posterior left ventricular branches from the other side. Regardless of the dominant artery, it is always the left coronary artery which supplies the major portion of ventricular myocardium.