A new code is proposed that can be used as a supplement to th ICHD code to describe single- and dual-chamber pacemaker operating modes. The code visually indicates the chamber(s) involved by means of a common base-line with indicators for atrial activity above and ventricular activity below. Letter indicators are used. For single-chamber units, an O represents the chamber in which no pacemaker activity occurs. P represents pacing, S sensing, IA inhibition by a sensed atrial event, IV inhibition by a sensed ventricular event, TA triggering by a sensed atrial event, and TV triggering by a sensed ventricular event. For example, a pacemaker mode described as PSIAIV/PSTAIV paces and senses both chambers; inhibits atrial pacing by a sensed event in either chamber; inhibits ventricular pacing by a sensed ventricular event but triggers ventricular pacing, after an assumed A-V delay, by a sensed atrial event. Thus even the most complex modes of pacing operation can be clearly and accurately described by means of this new code. Especially useful in teaching applications is the distinction of activity by chamber and the description of response in one chamber to activity in the other.