The mainspring is what the name implies. The "mainspring" of the clock. It is what supplies power to drive the gear train, which in turns drives the hands or the strike gear train. All wind up clocks will have at least one. If it's a striking clock it will have two. If it's a clock that plays music or chime's as the pros call it, it will have three. Get the picture? So if it has one winding hole it is a time only movement. A clock with two winding holes in it most likely will strike on the hour. Meaning that if it is 4 o'clock it will strike 4 times. Then moving on to the ones with three winding holes would mean it has three mainsprings. These are known as chime clocks. The most common type is the Westminster. It will play 4 notes on the 1/4-hour and 8 on the half and so on. Then on the hour it will play 16 notes and then count the hour. So as you can see it will have 3 mainsprings. One for the time, one for the chimes and the last for the strike. There is some cases that this won't be true. Boy I sure got away from the subject. Over years the mainspring can lose its spring tension. When this happens your clock might only run 5 days on a full wind when it should be an 8-day clock. This won't hurt anything but you will have to wind it more often. Mainsprings are not that expensive to buy and in some clocks a piece of cake to change. But in the majority of them it can be a real pain. 75% of them will require the movement to be disassembled. This is no big deal for the experienced repairman. For an amateur this can be a nightmare but It can be done. Most of then will go together two ways. The right way and the wrong way. The phrase that we use in the army a lot is "idiot proof". Most will go together one way. More on this later. If you ever hear someone in a clock shop tell you you've over wound you clock, turn around and walk out the door. If you wind a mainspring up till it stops that's as tight as its going to get. If you put a wrench on the key then you can over wind it. What happens is when you wind a clock up tight the mainsprings are wrapped tight against each other. If it hasn't been cleaned lately, like in the past 5 or 6 years the lube between the wraps of the mainsprings get gummy or not there at all. They have to be able to slip against each other to let down. This is the process that powers the clock. If you wind your clock tight and don't hear a big bang it's probably time for a cleaning if it has a hard time running or the strike is slow. If you do hear a big bang the spring ether broke or slipped off the winding arbor. If this happens you will probably have bent teeth and pinions. This is not good. Hope you have your checkbook handy. Just kidding.