And when they shut close, that is the second sound that we normally hear in our lub dub. So lub is when our bicuspid and tricuspid valves close, but the dub takes place when our two semi lunar valves close. We have the pulmonary and the aortic semi lunar valve. So the QRS complex is when our bicuspid and tricuspid valves close, but the T wave is when these two valves right here actually close. Now, what about this last wave we call the U wave. So the U wave is usually a very small peaked wave that follows our T wave.