I’d spotted Tern Key among the gallery of past offerings on Cubic Dissection and thought it looked rather nice. Designed by Goh Pit Khiam, this puzzle shares the same underlying principles as his Ternary Burr – both of which extend on the basics behind the Chinese Rings. While the Chinese Rings (and indeed Pit’s earlier Key Puzzle) is a binary sequence, Tern Key (and Ternary Burr – must try and find one of those at a reasonable price one day…) is based on a ternary, or base 3, sequence.
The puzzle is effectively a series of latches that must be released in sequence, but each latch forces you to effectively undo most of what you’ve already done – and there are three steps to releasing each latch in the case of the Tern Key.
Eric Fuller made 50 copies of Tern Key back in 2009. They’re manufactured using acrylic, nylon and stainless steel – so they’re really shiny and won’t ever get tired-looking. It is really nicely made and not only looks great, but the movements are all neat and crisp and positive as well – all 134 moves of them in the full solution. With all the clear acrylic, you can see exactly what you’re trying to do all the way through solving the puzzle – which finally sees the key piece being completed released and removed. Eric’s taken a lot of trouble to make sure that the bits of stainless steel used as pins and posts are all neatly polished and look terrific.
Puzzle-guru Dr Goetz Schwandtner has written a great paper on this class of puzzles, concentrating on Kugellager, which is a quinary (base 5) extension of the principle. The paper is available for download over here and you’ll find mention of all of the variants discussed in their appropriate sections in the paper. One of the eye-opening bits of the analysis is a graph showing how the complexity of the solution explodes super-exponentially as the number of latches (or rings) increases.