The aim of this work was quantitatively to establish the relationship between the plane that hosts the humeral head lateral margin (anatomical neck) and that of the capsular insertion. Eight cadaveric shoulders were used. These were dissected, exposing the humeral head margin and the root of the capsular humeral insertion to extract digitally their outlines using a mechanical 3‐d digitizer. The datasets of the digitized outlines were applied and the geometric planes they best fitted mathematically calculated. Vector analysis techniques were finally applied to the two planes to quantify the relationship between them. The humeral head margin is circular (± 2.2% of radius), having each of its outlining points on the same plane (within ± 1.5 mm.) The capsular attachment outlining points also insert on a plane (± 1.4 mm). The two planes are related to one another by an inclination of 14.5 ± 3.6°. The relationship described here would allow for in vivo prediction of humeral attachment of capsular structures by using radiological datasets of the anatomical neck. This would be useful in patient‐specific modelling to study and understand the glenohumeral ligament kinematics during clinical examinations and to plan surgical reconstructive procedures.