What are the rules of airfoil thickness from root to tip on tapered wings? I'm doing some research on wings and airfoils used in the 30s and 40s, and I'm trying to understand the rules about airfoil proportional thickness changes across the wing (root to tip) on tapered wings.

Many planes of that time used airfoils with thickness ranging from 15% to 9% (root to tip). (Source: https://m-selig.ae.illinois.edu/ads/aircraft.html)

So the question is: what are the rules for dimensioning airfoil thickness from root to tip?
Airfoil thickness at the root was chosen to obtain a good cantilever ratio: the bending moment is highest there, and you need to give it structural height. Also, you want to create most lift at the wing root, that is why chord, thickness and incidence angle are all highest there. All three tapering down towards the tip for structural reasons.

There is a bit more info in this answer.