The localization accuracy of the start of activity in the heart depends on electrode positions. Limited electrode systems have no or only a few electrodes on the back of the torso. The contribution of the electrodes on the back is investigated through a computer simulation of a current dipole placed in an in.nite conduction medium. The accuracy of localization is estimated by correlation coe.cient of potentials on the electrodes that result from the current dipole placed in a selected region in the heart and the potentials that result from the dipole placed in the vicinity. The results show that there exists an optimal location for an additional electrode placed on the torso for each position and orientation of the current dipole. It was also shown that there are some small number of advantageous electrode positions if all of them are allowed to be moved.