Abstract 1. 1. A theoretical analysis is made of the mirror image electrocardiograms which are often found at anatomically opposite positions across the heart. 2. 2. A cancellation technique is proposed whereby similar parts of presumed mirror patterns are cancelled leaving only the unbalanceable portion as a residual. 3. 3. A cancellation coefficient has been devised which permits quantitative evaluation of mirror pattern excellence. 4. 4. Use of a polar cardioscope is proposed for detailed study of electrocardiographic patterns over a lengthy observation period and is found valuable experimentally. 5. 5. A double exposure photographic method is utilized to establish cancellation axes visually within the body. 6. 6. The existence of deceptive “false” mirror patterns is pointed out and demonstrated. 7. 7. The importance of exact phase coincidence for cancellation is emphasized. 8. 8. True mirror patterns of good cancellation excellence are not rare and most patterns have good mirror images, but some noncancellable patterns are found.