To show relationships among planes and angles, A. H. Wheeler designed a series of what he called platform models. All of these had a rectangular base or platform, with other segments (often other rectangles) secured to it. These other pieces sometimes penetrated the platform.
This plastic platform model has a cream-colored base with an orange rectangle penetrating it perpendicularly. The second piece is held on the bottom by yellow supports. It illustrates perpendicular planes.
George A. Wentworth, Plane and Solid Geometry, Boston: Ginn, 1899, p. 261-262.
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