So we have the points shown in blue. We have point P and point R, and we have point S and point T. So let's discuss what each one of these segments and waves actually represents. Let's begin with wave P or the P wave. So let's recall where the heart actually begins, where it generates that electrical signal. So within the right atrium, within the upper wall of the right atrium, we have a collection of specialized cells that collectively are called the sinoatrial node, or simply the SA node. So this is where the SA node is located.