I discovered something a little puzzling recently; it’s surprisingly difficult to get a large picture frame with the correct aspect ratio to fit a print from 35mm film.
The standard size for photographic prints from 35mm film is 4″ x 6″, which translates to an aspect ratio of 2:3. However, frame sizes larger than this tend to have the 4:5 aspect ratio of medium-format film, e.g. 8″ x 10″ or 16″ x 20″.
I also discovered recently that digital cameras have yet another different aspect ratio, 3:4 (I found a useful discussion of digital vs film photo dimensions on the ACDSee website, which is often a useful resource for imaging information).