The trapezoid layer is created using information from both the
cylinder and the first
triangle layer. The coordinates used to create the bottom of the
first triangle layer are the same coordinates for the top of the
trapezoid layer, while the bottom part of the trapezoid layer connects
to the top of the cylinder. The cylinder is sectioned off into twelve
vertical strips leaving every other one blank in order to simulate the
idea that a space represents a window. Thus, in order to finish the
trapezoid and complete the layer, twelve equally-spaced vertices lying
on the circle at the top of the cylinder are needed. The measurements
from the side-elevation establish the height
of these points. The radius is the number measured as the radius of
the cylinder. So, using the same method that is used to create the
triangle layers, one is able to compute the
twelve desired points.

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