Square-Bracing Problem/Non-Crossing Rods

Classic Problem

Let there be an unlimited supply of rigid rods of unit length which may be joined together at their ends with a hinged joint.

Let $4$ of these rods be used to construct a square.


How many more of these rods are needed to make the square rigid?


Rods are not allowed to cross.


Solution

This solution uses $23$ rods:




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Sources

  • 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $23$
  • 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $23$
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