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 ECG labeling quiz draws one normal heartbeat as a trace on a grid, with three beats in a row under it. The waves come first, then the intervals and segments. It is a study chart with names only.
Read the full guide
What the heart does at each wave
The atria depolarize during the P wave. The impulse then passes through the atrioventricular node, which is the flat PR segment. The ventricles depolarize during the QRS complex and repolarize during the T wave.
Segments and intervals
The PR segment and the ST segment are flat pieces of the trace between two waves, and each has a short bracket over it. The PR interval and the QT interval take in whole waves, and each has a bracket under the trace. The chart is a scheme with no scale.
On the big beat, brackets above the trace are segments. Brackets below it are the two intervals and the QRS complex.
The R-R interval is on the lower strip, because it needs two beats.
Questions and answers
What are the parts of an ECG wave?
One normal beat has a P wave, a QRS complex made of the Q, R and S waves, and a T wave. The PR segment and the ST segment lie between them, and the PR interval and the QT interval span several of these parts.
What does the QRS complex show?
The depolarization of the ventricles. The Q wave is the small dip before the tall R wave, and the S wave is the dip after it. The three are usually taken together as one complex.
What is the difference between the PR interval and the PR segment?
The PR interval runs from the start of the P wave to the start of the QRS complex, so it includes the P wave. The PR segment is only the flat piece between the end of the P wave and the QRS complex.
Is an EKG labeling quiz the same thing?
Yes. ECG and EKG both stand for electrocardiogram, so it asks for the same waves, segments and intervals.
Why is there no U wave on the chart?
A small U wave sometimes follows the T wave, but it is often not seen, so this chart of one normal beat leaves it out.