At this point in Book VI it is customary to introduce the dihedral angle.
The locus of a point equidistant from the faces of a dihedral angle is the plane bisecting the angle.
The reason is that the flexible covering will be bowed back by the wind, forming an approximate "dihedral angle."
In determining the center of gravity, the bird was frozen in the soaring position, its wings making a dihedral angle of 150.