Annals of Cardiothoracic Surgery 8(3), 313–321 (May 2019), DOI 10.21037/acs.2019.04.16. Technical anatomical exposition by the cardiac surgeons named in the primary byline. General considerations and Figures 1–2 distinguish the basal reference, crown-shaped attachment and sinotubular boundary. Figure 1 identifies separate measurement levels in an echocardiographic view.