Schematic demonstration of the fibromuscular membrane dividing the left atrial cavity into two chambers. Possible ways of communication between the proximal left atrial chamber, right atrium and the distal chamber (true left atrium) are demonstrated. The communication may lie between the proximal chamber and the right atrium (A) and/or between the right atrium and the distal chamber (B) and/or through the fenestrations on the membrane (C). SVC: superior vena cava; CS: coronary sinus.