In this article we aimed to establish that interatrial block exists as an anatomical-electrical entity, which should be considered a true block. Interatrial block presents with different degrees as other blocks in the conduction system. It shows a correlation with the left atrium size, however, it can be seen in patients with normal atrial size too. Interatrial block is strongly associated with atrial arrhythmias and it could be considered a predictor of cardioembolic stroke. Interatrial block is an expression of atrial electrical remodeling and dysfunction. IAB can be transient and in certain clinical circumstances, may be reversible. The contribution of endocardial mapping has increased our knowledge of the anatomy and pathophysiology of interatrial block. Magnetocardiography could be a possible non-invasive procedure to further investigate this entity. The interatrial block classification should include first, second and third degree or alternatively, in order to simplify the terminology: partial or advanced. The P wave morphology should always be taking into consideration when diagnosing this condition. Finally, without the initial description of interatrial block made by Dr. Bayés de Luna, it would be impossible to understand interatrial block as an anatomical and electrical substrate for atrial arrhythmias. It is our opinion that this represents a major contribution to the knowledge of electrocardiography and electrophysiology, and makes commendable that this arrhythmic syndrome should be called «Bayés' syndrome»