As you can see by the size scales, the sizes are close, and they intersect at size 3 so a size 3 clock key will fit both a standard and metric clock just as it should. The clock key should fit perfectly over the post with no ‘play’, or wiggle room. It should be very snug if a proper fit, but as you can see in the charts, key No. 2 between the 2 sizes is a difference of .05 which is hardly even noticeable, but you will see some play. This also means you would need to be careful when purchasing a clock key because if your clocks winding post is a standard system 2 (2.8mm), then a metric system no. 2 key will not work because it would be too small to fit over the post being .05mm smaller than the post itself. The best thing to do if you are unsure of the size you need is to measure the clock’s winding post that you need a key for, and use these guides to help with your purchase. If you purchase the correct number, like I said above, it will be a perfect fit with no movement on the post at all, but both systems move up in such small increments that the next 2 sizes will also work, so if you need a standard No. 5 key, a standard No. 6 and 7 key would work as well, though there would be some movement between the post and the key if not the exact number.

You will also notice that in some auctions a key will be numbered on the body of the key itself, but does not actually fit the dimensions of that number on either size scale, so you may see a key that is numbered with a 9, though it is actually a metric 7 and standard 8. I do not know why this is though I am looking into it, but please note that whatever size any key is listed as, that number is based on the true size of that keys square hole.

If you have any other questions or need any additional clarification, please do not hesitate to contact me as I am always more than happy to help as best I can. Thank you again for checking out my items, and have a great day!