I was reading this article this morning:
And I noticed the Moots Mounts on the rear triangle (they can be used on a fork as well).
One picture worths more than 10.000 posts.
(Picture taken from http://www.vintagemtbworkshop.com/1984-moots-mountaineer.html)
It’s incredible how simple this has been engineered back in 1984, with the clear intention to use different wheel size on the same bike (course, you may have the need to use a brake booster).
Just think about the “new” gravel craziness with it's need to switch your wheels (between 700c and 27,5) while Moots was doing that, for the same purpose, more than 30 years ago.