As I advised in my previous answer, a 38mm band is not designed to fit a 42mm Apple Watch. Nor is a 42mm band designed to fit a 38mm Apple Watch. A 38mm or 42mm Apple Watch band will fit an Apple Watch of the same case size, regardless of its size. I recently ordered a new Apple Watch band from Amazon.
Since I have a 42mm model, it only made sense to order the one that was made for this particular model. I chose the one I liked, added it to my Amazon cart and made the payment. I was told that I can return the item once it has shipped, which was a bit sad since I wanted to use the new watch band at an upcoming event. The watch comes in a new case size of 41 mm and 45 mm, moving away from the 40 mm and 44 mm size that has been the standard of the last generations of Apple Watch.
While you can also use a 42mm band on a 38mm Apple Watch model, the ends of the band will stick out of the watch, which may seem odd. A few days later the watch band arrived, and instead of returning it right away, I decided to open the package and see if the smaller band fits my 42mm Apple Watch. But many of them no longer include options for the 38, 40, 42 and 44 mm size options for all bands. Usually, any watch band will be compatible with any Apple Watch, however, there are some differences in which Apple Watch band you can use depending on whether you have opted for the large or small version of your Apple Watch generation.
As for the Apple Watch Series 4, it has been updated to a new size (40 mm and 44 mm instead of 38 mm and 42 mm) and the old watch bands fit the new ones. Getting a larger Apple Watch band basically ensures that it will work with previous wearables that were the larger size option of their respective generation. Which means that when you put the 38mm strap on a 42mm watch, there will be small but noticeable gaps on either side of the ends of the band (as indicated by red arrows in the image above). Solo Loop and Braided Solo Loop bands are only compatible with Apple Watch SE and Apple Watch Series 4 or newer.
A 42mm band on an Apple Watch will fit an Apple Watch of the same case size as any 38mm or 42mm Apple Watch band. While it may seem at first that Apple has unceremoniously eradicated support for all previous models, don't worry, it didn't. To reiterate, bands made for the smaller of the two new sizes work with previous smaller Apple Watch models, and vice versa. If the strap is properly installed, it will not slide freely until you hold down the strap release button.
Apple Watches have two screen sizes, 42 mm and 44 mm, but both models have the same mounting slots for the watch straps. In case you didn't know, they're only compatible with Apple Watch SE, Series 4, and newer models.
