We try to remember to include some good old "Normal Sinus Rhythm" strips from time to time.  Teachers often have large collections of strange and unusual strips that their colleagues have saved for them.  But, then they find themselves resorting to electronic rhythm generators for samples of "normal".  Here is a strip from a healthy, 23-year-old woman showing NSR.  The rate is 65 bpm, QRS duration 76 ms, PRI 136 ms, QTc 410.  There are no abnormal ST segments or T wave changes.  There are very slight rate changes from beat to beat, and the P waves appear to change morphology a bit.  This can be due to the patient's breathing movements, and we would not delve too deeply into this in light of the fact that this is an ECG from an asymptomatic young ECG student.  Absolute precision would come from an generator, but rarely from a human being. This is a good strip to teach rate and interval determination.