The Apple Watch is available in 5 different sizes: 38, 40, 41, 42, and 44mm. The straps for the Apple Watch 38/40/41mm fit all these watches. You can also use these straps for the Apple Watch Series 7 41mm. So, if you have the Apple Watch 38mm and you want to buy a 40mm strap, you can. The watch strap attachment still fits the Apple Watch 38mm. This also applies the other way around. You can also wear Apple Watch 38mm straps on the Apple Watch 40mm. You can choose from a lot of watch straps for your Apple Watch.