P waveThe atria depolarize while the pen draws the P wave. It is the first wave of the beat.
QRS complexThe ventricles depolarize during the QRS complex. It is the Q, R and S waves taken together.
T waveThe ventricles repolarize while the pen draws the T wave. It follows the S wave.
R waveThe R wave is the upward stroke of the QRS complex. It is the tallest wave on this chart.
Q waveThe Q wave is the small dip before the R wave. It begins the QRS complex.
S waveThe S wave is the downward stroke after the R wave. It ends the QRS complex.
BaselineThe baseline is the flat line between beats. It is also called the isoelectric line.
Intervals and segments 6
PR intervalFrom where the P wave starts to the QRS complex. The impulse travels from the atria to the ventricles in this time.
PR segmentThe PR segment is the flat piece after the P wave. The impulse is passing through the atrioventricular node.
J pointThe J point is where the QRS complex ends. The ST segment begins there.
ST segmentThe ST segment joins the QRS complex to the T wave. The ventricles are depolarized all through it.
QT intervalIt spans the QRS complex, the ST segment and the T wave. The ventricles depolarize and repolarize in this time.
R-R intervalThe R-R interval runs from one R wave to the next. It is the length of one beat of the ventricles.
How this list was compiled: Every name and detail line was checked on 2026-10-02 and kept only where two independent sources give it, read for facts only: (1) Wikipedia, Electrocardiography (the table of waves and intervals) and QRS complex, https://en.wikipedia.org/wiki/Electrocardiography and https://en.wikipedia.org/wiki/QRS_complex; (2) Klabunde, Cardiovascular Physiology Concepts, Electrocardiogram, https://cvphysiology.com/arrhythmias/a009; (3) the ECG Learning Center of the University of Utah, lessons 1 and 3, https://ecg.utah.edu/lesson/1 and https://ecg.utah.edu/lesson/3; (4) Life in the Fast Lane, ECG library, the pages Q wave, R wave and PR segment, https://litfl.com/ecg-library/basics/. All wording is our own. Licence: own work
The drawing: Drawn for this site on 2026-10-02 by script from plain geometry; nothing traced. The trace is schematic. Licence: own work
How it works
This unlabeled ECG diagram is one normal heartbeat drawn as a trace on a grid, with a strip of three beats under it. No part is named on it: thirteen wait for a name, and the answer key prints separately.
Read the full guide
What to write on it
On the trace itself: the P wave, the Q, R and S waves, the T wave and the baseline. Marked by a bracket: the PR segment and the ST segment above the trace; the PR interval, the QRS complex and the QT interval below it; the R-R interval on the lower strip. The dot is the J point.
Name the five waves first: every bracket begins and ends at one of them.
The five waves run in alphabetical order from left to right: P, Q, R, S, T.
Questions and answers
Why are the ECG waves called P, Q, R, S and T?
Willem Einthoven gave the deflections those letters in 1895, and they are still in use. An earlier, uncorrected trace had been lettered A, B, C and D, so new letters kept the two apart on one graph.
Is an unlabeled EKG diagram the same thing?
Yes. ECG and EKG are two abbreviations for electrocardiogram, the recorded tracing, so both names ask for this drawing.
What is the J point on an ECG?
The point where the QRS complex ends and the ST segment begins. On this diagram it is the dot just after the S wave.
Does the diagram show a real recording?
No. It is a scheme with no scale: the Q, R and S waves are drawn wider than they are so each has room for a label, and no time or height is printed. It teaches the names only.