Aims: The anatomic substrates for atrioventricular nodal re-entry remain enigmatic, but require knowledge of the normal arrangement of the inputs and exist from the atrioventricular node. This knowledge is crucial to understand the phenomenon of atrioventricular nodal re-entry. Methods and results: We studied 20 human hearts with serial sections covering the entirety of the triangle of Koch and the cavotricuspid isthmus. We determined the location of the atrioventricular conduction axis and the connections between the specialized cardiomyocytes of the conduction axis and the adjacent working atrial myocardium. The atrioventricular node was found at the apex of the triangle of Koch, with entry of the conduction axis to the central fibrous body providing the criterion for distinction of the bundle of His. We found marked variation in the inferior extensions of the node, the shape of the node, the presence or absence of a connecting bridge within the myocardium of the cavotricuspid isthmus, the connections between the compact node and the myocardium of the atrial septum, the presence of transitional cardiomyocytes, and the ‘last’ connection between the working atrial myocardium and the conduction axis before it became the bundle of His. Conclusion: The observed variations of the inferior extensions, combined with the arrangement of the ‘last’ connections between the atrial myocardium and the conduction axis prior to its insulation as the bundle of His, provide compelling evidence to support the concept for atrioventricular nodal re-entry as advanced by Katritsis and Becker.