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 platform, with other segments (often other rectangles) secured to it. This tan paper platform model shows the intersection of four planes. One serves as the platform and the other three intersect in a line perpendicular to the platform. The third plane (which is represented by a triangle between two other triangles) is movable. The line at the base of this central triangle is perpendicular to the line joining the two upper triangles and, when moved to intersect the line drawn on the base, perpendicular to that line. Hence a line perpendicular to two llines.
Compare MA.304723.650,1979.0102.183,1979.0102.265,1979.0102.266, and 1979.0102.267.
For a related pattern, see 1979.3002.109.
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