Somewhere, sometime in history someone realized that if you tie the flat surfaces of two thin bits of reed together, put these blades between your lips, and blow, the reeds will vibrate and make a squawking sound.
At school we did the same thing with tissue paper over a comb. This principal was later embodied in many musical instruments to provide them with a voice. Clarinets and the like use just one flappy reed against a fixed lip-piece and other instruments use two flaps of reed which vibrate against each other. But the bagpipes are rather special. With other instruments you have to stop the music periodically to allow the musician to take a quick breath so he can go on playing without expiring. The bagpipes have a lung of their own. The bag is just that. It is an external lung which the player props in under his arm and squeezes steadily to provide the airflow needed to maintain the tune even whilst the music maker is taking in fresh air. He then blows more air into the bag through his blowpipe to keep it inflated. This is tricky to do in practice but when perfected it works very well. Actually, they discovered that it works so well that they could attach three more reeded pipes to the bag and get them all to sing in harmony. The piper can of course only play one tune at a time. So the reed instrument taht is being given voice by air from the bag, has a number of holes down its length to enable the piper to play different notes. This tube is called the chanter. The other three reeded pipes each simply sound off on a single monotonous note. They are called drones for obvious reasons. Two of them are tenor drones and they each produce a monotone which is exactly an octave below the ‘low-A’ note of the chanter. The third and longer, bass drone produces a note that is yet another octave lower. For those not musically informed, two notes an octave apart will be in perfect harmony. For each flap of the reed in the lower tuned pipe, the reed in the shorter pipe will flap twice. The combined tone we then hear is acoustically interesting because we can hear that the two notes are very different but the combination pleases us because they beat in time with each other. To understand a similar thing in the visual world, imagine the following. Two soldiers march side by side. They move forward at the same speed but one takes two short paces for each long pace that the other takes. Their movements are patently different but they are clearly still in step. They can be said to be synchronized, or in harmony. More on bagpipes at another time.