Clock Key Number 1 / 2.50mm Selecting the right key size The number sizing systems were developed many years ago and the number on the key corresponds to a size on a particular chart. This system becomes an issue when you find out there?s at least three different scales/charts (American, Swiss, etc...) and all are slightly different, with measurements taken from different points on the arbor. Which means that a No 6 key made by one manufacturer may not be the same as a No 6 by another. An example of this is a Number 6 key using the American system is 3.6mm and using the Swiss system is 3.75mm. The answer to this problem is to measure the arbor The best way is to measure the winding arbor (the steel shaft the key fits on to). If you have a set of vernier callipers get them on the end of the winding arbor and you will get an accurate reading in millimeters. Then its just a matter of selecting the correct key. This is the best way to get a key that fits your clock. Note: all of the clock keys that we list are measured using a clock key gauge that has been verified true. We have alot of clock related items so check out our other auctions too. We do combine shipping and will always refund any excess shipping charges. Thanks for your interest and/or business. Tim