Abstract: An endocardial electrode assembly of three or more spring legs to be inserted into the heart, sets of electrodes on respective spring legs comprising distinctive geometrical patterns, over the ventricle-contacting span of the legs, such that the active sets of electrodes provide recognizable patterns when within a ventricle of the heart and viewed with a fluoroscope, the respective locations of at least some of the electrodes of these patterns being staggered relative to electrodes on other legs in the manner to reduce lateral clumping, thus to permit the numerous electrodes to be housed in a catheter sized to be conveniently introduced into the heart.