The width. I use this everyday for several hours a day as I work from home. Initially bought one of these for recording acoustic guitar. It only works when there is a text field you want to type in. I bought size 1-2 Little Kid. That tiny distance managed to put several distinct scatches on the plastic. One is the fit, but like I have just implied, I think that is an isolated thing with my ears on account that they could be oddly shaped. Le tapis où se déplace la souris, ne bouge jamais. No issues and, so far (less than 24 hrs), works fine and great looking too :)