Paired anterior cardinal veins form common cardinal veins with paired posterior cardinal veins, draining centrally into the sinus venosus (sinus horns) as depicted (top). Paired anterior cardinals soon form an anastomosis between them; the connection grows from the left to the right anterior cardinal vein to form the left brachiocephalic (innominate) vein (bottom). The left anterior cardinal vein distal (cranial) to the anastomosis becomes the ‘left internal jugular vein,’ while the left anterior cardinal vein proximal to the brachiocephalic anastomosis regresses/atrophies to become the base of the ‘coronary sinus’ of the heart as displayed. The right anterior cardinal (precardinal) vein proximal to the right brachiocephalic vein forms the superior vena cava (SVC) with the common cardinal, and terminal/proximal segment of the posterior cardinal (postcardinal) vein. The figure was reproduced with permission from Lee BB: Venous embryology: the key to understanding anomalous venous conditions. Phlebolymphology 2012, 4:170–181.