| 摘要 |
Two tetrahedrons are fixed together at a common joint and provided with a seat fixed between faces of respective tetrahedrons and parallel to a support plane formed by two joints meeting the common joint at the common joint. For equilateral tetrahedrons which meet at an angle of 109 DEG 30' between faces, plural seating can be provided by fixing three or more tetrahedrons together in a variety of configurations having a common support plane and seats parallel thereto.
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