Historical overview

Little is known about the early history of interlocking puzzles, but they were certainly produced both in Asia and Europe as early as the 18th century. Six-piece burrs were shown as early as 1803 in the Bestelmeier Toy catalogs. But it wasn't until Edwin Wyatt published Puzzles in Wood in 1928 that a book devoted to many interlocking puzzles was available. Puzzles of this type used to be known as "Chinese" puzzles, probably because they were mass-produced in the Orient since the early 1900s, but there does not appear to be any evidence that the idea originated there. Nowadays they are commonly referred to as "burr" puzzles. Wyatt introduced the term because the puzzles looked like a seed burr.

By far the most familiar burr is the six-piece burr. The six-piece burr is actually a large family of designs, since the designer has a wide choice of how to notch each piece. Several versions have been patented and manufactured. The earliest US patent is No. 1,225,760 of Brown, dated 1917, with several others following shortly thereafter.

Most toy and novelty stores have a few burr puzzles on their shelves. Unfortunately these are often the uninspired, time-worn versions with sliding keys and internal symmetries. The puzzle has suffered from this tarnished image, and to make matters worse, inventors have tinkered with bizarre embellishments like cords through holes to give the basic puzzle their twist.

In 1978, Bill Cutler has returned to the basic design and published a paper describing the solid six-piece burr completely [Cutler78]. He has shown that there are 25 possible notchable pieces to make solid six-piece burrs and that they can be put together in 314 ways. He also showed that there are 369 general pieces usable to make solid burrs and that they can be put together in 119,979 ways.

More recently, Stewart Coffin, Bill Cutler, Philippe Dubois, and Peter Marineau have come up with higher-level six-piece burr designs that are not solid and in which several pieces must be moved before one can be taken out. Stewart Coffin has enhanced the art of burr designs into abstract geometrical forms. His book The Puzzling World of Polyhedral Dissections contains close to 100 exquisite designs of burr and other wooden puzzles.

From the late 1980s to the mid 1990 Bill Cutler and others undertook a complete analysis of all six-piece burrs [Cutler94]. Original estimates by Cutler assumed it would take about 62.5 years to run the analysis on a PC AT. With some help from others who run Cutlers programs on faster and bigger machines, total calculation time was cut down to 2.5 years. From this analysis we now know that there are roughly 35.65 billion ways to assemble burr puzzles pieces (71.3 billion if mirror images are counted also). Of these 35.65 billion logical assemblies 5.95 billion can be taken apart. The highest level found was a level 12 puzzle. Unfortunately it is not unique, i.e . it has more than one assembly, most of which are of a much lower level. The highest level unique six-piece burr is of level 10 if the pieces are 8 units long and level 9 if the pieces are 6 units in length. If all pieces are notchable, the highest level is 5 for a unique burr.