Usually asymptomatic cardiac disease with AV bypass fibers that conduct in an antegrade direction; majority of accessory pathways conduct in the left wall. Subtypes are a function of the direction of the initial delta wave on ECG. Type A-ventricular activation occurs 1st in inferior-posterior region of the left ventricle, with anteriorly directed delta wave and variably appearing Q-wave in leads II, III, and aVF; type B-ventricular activation occurs first in inferior-posterior region of right ventricle, with delta wave directed posteriorly and to the left with a negative initial deflection and rS or qS patttern in V1; type C-ventricular activation occurs first in posterior lateral region of left ventricle, with positive delta wave in V1 and negative or isoelectric delta waves in leads V5 and V6.