Natural slate laid to a new roof, with a lead valley running down between the two slopes.
Slate wants a clean, even gauge across the full width of the slope. The courses and the joint pattern are what anyone actually notices from the ground, so they get set out first and held to all the way up.
The valley is lead-lined and dressed under the slate on both sides. Where two roofs meet is where water collects, so there is nothing there to fail.