1 small box = 1 mm = 40ms = .1mV amplitude

1 large box = 5 mm = 200ms = .5mV amplitude

Normal PR = 120-200 ms (3 to 5 small boxes)
.... prolonged in incrased K or Mg, first degree heart block (AV node block)
.... shortened in pre-excitation via accessory pathway (Wolff-Parkinson-White sydnrome)

Normal QRS = 60-100ms (3 small squares or less)
...widened in increased K or Mg,

Q wave should be less than 1/3 the height of the R wave.  If its higher.... think Myocardial Infarction.

T waves
-peaked --> hyperkaelemia
-negative... normal in lead aVR. (also possible in aVL, aVF, III) Lead V1 may have a positive, negative, or biphasic T wave.
..otherwise, can indicate Coronary ischemia, LVH.

U waves -- come right after T waves
-in hypokalemia, hypercalcemia, thyrotoxicosis