The pulmonary veins develop from the respiratory diverticulum, which arises from the primitive foregut. The developing veins are located posteriorly to the left of the septum primum. The rudimentary pulmonary vein becomes incorporated into the atrium of the developing heart (Bordoni and Matthew 2020). The oblique vein (also called the vein of Marshall) is an embryological remnant that persists into adulthood in many hearts (Żabówka et al. 2020). In others, it is obliterated and becomes the ligament of Marshall. The vein of Marshall is frequently used in ablation procedures to treat atrial fibrillation as it contains myocardial tissue, making it continuous with the musculature of the coronary sinus (Fig. 43.1).