In an interlocking puzzle, one or more parts hold the rest together or the parts are mutually supporting. The aim is to completely disassemble and then reassemble the game. Examples of this kind of puzzles are the well-known Chinese Wood Knots.
Both assembly and disassembly may be very difficult; unlike assembling puzzles, these are not so easy to separate.
The difficulty level is usually evaluated in terms of the number of movements required to remove the first part from the initial puzzle.
The image on the right shows one of the most renowned representatives of this category: the Chinese Wood Knot. In this particular version, designed by Bill Cutler, 5 movements are needed before the first part could be removed.
Recorded history of these puzzles goes back to the beginning of the XVIII century: in 1803 a catalog of "Bastelmeier" contained two puzzles of this kind. Even Professor Hoffman's famous Puzzle Book contained two interlocking puzzles.
In the early XIX century the Japanese conquered this kind of puzzle market. They developed a large number of games with all kinds of figures: animals, houses and other objects, while development in the Western world has mainly revolved around geometric figures.
By the help of recent computers, it is now possible to analyze complete sets of games. This process started with Bill Cutler and his analysis of all Chinese Wood Knots. From October 1987 to August 1990 all the 35 million (35.657.131.235) different variants were analyzed. The calculations were made consecutively from different computers and would have taken a total of 62.5 years on a single computer.
With the Chinese Cross' many figures, the difficulty level has recently reached 100 movements in order to move the first piece, which challenges human abilities. The peak of this development is a puzzle where the addition of few pieces doubles the number of movements.
However, computerized analysis led to another trend: as PC programs still fail to analyze the rotation of puzzle pieces, there has been a tendency to design puzzles with at least one rotation. So they can be solved by hand.
Before RD Design Project publication in 2003 by Owen, Charnley and Strickland, puzzles with no right angles could not be analyzed efficiently by computers. Although Stewart Coffin was creating puzzles based on the rhombic dodecahedron since the 1960s. They used both six and three-sided strips. This kind of puzzles often has extremely irregular components and come together in a normal figure only at the last point. Furthermore, 60° angles allow design where even different pieces need to be moved at the same time.
The "Rosebud Puzzle" is a typical example: 6 pieces must be moved from a lateral position towards the center of the completed object, where only corners touch each other.
Some examples are: