My question relates to the difference in shape (not thickness) between cedar shingles and cedar tapersawn shakes, both of which are sawn (unlike standard shakes which are split), and therefore have a smoother surface than a split shake.

I think the shingle is a right-triangle and the tapersawn is an isosceles triangle. Is that right?

I am trying to understand which specific difference between a shingle and a tapersawn shake makes it unacceptable to interlay felt with cedar shingles but acceptable to interlay felt with tapersawn shakes.