An ECG is a graph that shows the electrical activity of the heart. To understand this graph, it is key to understand the way the electrical activity travels through the heart. It starts in the sinus node, which contains cells that are able to initiate electrical signals. From the sinus node, the electrical signal travels through the walls of the atria to the AV-node. As the signal travels through the cells of the atria, it instructs these cells to contract. On an ECG, this is visualized as the P-wave. From the AV-node, the signal travels through the bundle of His to the bundle branches, activating the contraction of the cells of the ventricles. This creates the QRS-wave on an ECG. The electrical activity fades away after both atria and ventricles have contracted. However, after the QRS-wave a third wave is visible on the ECG. This represents the electrical activity that is generated by the heart muscle when it recovers and prepares itself for the next contraction.