Which end of wood shake siding goes down the thick end or the thin end?

Thick.

Thanks for the confirmation that is what I thought!

If you use Hardie concrete shingles, they go with the wide end down, they are the same thickness throughout.

Be sure that seams of shingle are offset by about 2''. This will help to seal water out.

If doing new and not matching existing would recommend an under course or triple over lap.

Chuck