In this installment of our journal’s teaching rounds, Debruyne et al1 provide an outstanding learning experience constructed on simple and logical teaching steps of what at first glance seems to be a complex and partially random observation. Their case and the findings they discuss highlight the difficulties in specifically targeting 1 of 2 structures living together on the true interatrial septum Article see p e73 A true septum is a partitioning wall shared between 2 structures. The true interatrial septum is a relatively limited structure, which in the periannular region is referred to as the midseptum. The fossa ovalis and this midseptal region are the only places where one can penetrate and can create an intraconnection between the 2 atria. More superiorly, the right and the left atria separate from each other with intervening fibrous tissue and pericardiac fat. This is why transseptal puncture across the superior limbus (above the fossa) necessarily exits the heart before reentering the left atrium. More caudally, the coronary sinus and the pyramidal space constitute the so-called posterior septum. This region, however, is not a partition between the right and the left atria but a separate posterior pyramid-shaped space. The midseptum is the place of residence for the compact AV node. Although accessory …