A lot of people have trouble with this miter. To understand it take a board and cut it in half. Take one piece and cut the plumb angle on it. In other words hold it at the angle of the roof and cut for plumb.
Now take the rake piece and hold it at the roof pitch and place the square cut piece next to it. You will notice that the lengths of the cuts are different, ergo the molding sizes must be different. This is illustrated in "Circular Work in Carpentry and Joinery" by George Collins.
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