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Each cube must bear a 0, 1, and 2. This leaves
only six faces for the remaining seven digits, but fortunately the
same face can be used for 6 and 9, depending on how the cube is
turned. The illustration shows 3, 4, 5 on the right (red) cube, and
therefore its hidden faces must be 0, 1, and 2. On the left (blue)
cube one can see 1 and 2, and so its hidden faces must be 0, 6 (or 9),
7, and 8.
John S. Singleton from England had patented the two-cube calendar in
1957/8 (British patent number 831572), but allowed the patent to lapse
in 1965. |
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