A 58-year-old woman presented with palpitations. A 12-lead electrocardiogram demonstrated sinus rhythm (SR) with premature contractions displaying a right bundle branch block (RBBB) morphology and a superior axis (Figure 1). Holter monitoring further revealed post-extrasystolic PR prolongation (Figure 2). The PR interval prolongation following interpolated premature complexes is an important clue to the origin of the premature beats. Both concealed conduction (CC) and dual pathway physiology represent important electrophysiological characteristics of the atrioventricular node (AVN). The CC (retrograde, concealed incomplete penetration of the AVN by the premature complexes) is defined as the partial penetration of a cardiac impulse into any component of the conduction system (i.e., the AVN or the His-Purkinje system) without resulting in a directly observable response (Figure 3). Instead, it modifies subsequent conduction by altering tissue refractoriness. The manifestations of CC can be summarized as: (a) conduction prolongation (Figure 4A), (b) failure of impulse propagation, (c) facilitation of conduction by "peeling back" refractoriness, and (d) pauses in the discharge of a spontaneous pacemaker (Figure 4B). Another clue is the similarity of the QRS morphologies between the initial part of the premature complexes and SR (Figure 5). Since the precordial leads used today are considered unipolar leads in that they measure voltage at a given location relative to approximately zero potential, these leads consist of a single active lead and an indifferent electrode, providing accurate information about the direction of electrical activation. The right bundle does not influence the time of arrival of activation to the lateral left ventricle (LV), because activation of the LV occurs via the left septal branch of the left bundle. Therefore, RBBB is characterized by an initial steep upstroke in the QRS complex on a unipolar electrogram, similar to SR, due to early septal activation since the LV has the same initial forces. The similarity of the initial QRS forces between SR and premature complexes indicates comparable early septal activation, supporting a supraventricular origin, with premature atrial contractions or premature Hisian contractions (PHC) as possible sources (Figure 4). The observed PR prolongation after premature beats (Figures 1 and 2), together with the QRS force similarity (Figure 4), suggests retrograde AVN penetration from an infranodal origin (Figures 3 and 4), leaving PHC as the most likely diagnosis, which was also confirmed by three-dimensional mapping (Figure 6).