This results in a perforation hole at each corner. These differences are usually only important to the specialist or advanced collector. I am sure there may be others. Perforations are measured by counting the number of holes per two centimeters. This would be a very tedious project if we had to count them for each stamp.
Gauges come in two types.
The first type are the ones you usually get free and they are the ones with little black spots. To use this first type, you just start at one side and move the stamp up and down until the spots fill all the holes. The second type of perforation gauge you usually have to buy and has lines that move from close together to further apart as one goes from one end to the other. These gauges are usually more accurate, but they require a little more skill to use. You read the perforation gauge value off either side at the point where the lines all line up with the holes.