The Apple Watch features a special attachment system for watch bands, so you will need to make sure any watch band you buy is designed specifically for the Apple Watch. The good news, however, is that bands are compatible across the respective sizes of every Apple Watch ever made, meaning that a band designed for the smaller 38mm Apple Watch Series 1 will also fit the 40mm Apple Watch Series 6 with no problems, and the same is true for the older 42mm Apple Watch models versus the newer 44mm versions.