A form-fit, form-locking or form-closed connection is a type of mechanical connection between two parts (example: screw and screwdriver), wherein these parts due to their forms interlock and block each other along at least one defined linear or rotational direction.
Form-fit connections are created by the interlocking of the connecting components. For example, the lid cannot slip sideways away from the pot because both interlock at the edge. On the other hand, a round lid can be rotated while sitting on the pot, because there is no form-fit against the rotation. Towards the bottom, the lid has a stop against the pot. This is a "half form-fit" because upwards it can be removed.
A form-fit acts via the geometric contact of two effective surfaces, and the effective forces are transmitted as normal forces to the effective surfaces of a driver part (surface pressure and Hertzian contact stress). Typically, some manufacturing-related tolerance occurs in the connection during form-fit.
In a form-fit connection, one connection partner blocks the movement of the other. Such "blocking" occurs in at least one direction. If a second pair of surfaces is arranged opposite, the opposite direction is also blocked.
Description.
Is a type of joint between parts, used to hold them together. takes advantage of their geometry. That is, unlike friction fasteners, which rely on clamping force, form-fitting fasteners use, as stated, the shape of the parts themselves to prevent them from separating. Joints can be fixed, articulated or other types. 
Examples: 
In solid assemblies, the forces that can act can be diverse. Often, in addition to the relative shapes, there are other effects (friction, elastic forces, adhesives, etc.) that must be taken into account for correct operation. For example, two Lego pieces fit together with a little interference. The plastic is sufficiently elastic and provides a small anchoring force. A balder assembly would not offer any anchoring effort and the stability of assemblies with hundreds of pieces would be very precarious. 
Form-fit closures can be observed in everyday objects and tools from primitive times to more advanced applications.
Made up of three pieces: two arms (wood or plastic) and a spring with levers. The spring is press-fitted by inserting the levers into the slots in the arms. The spring is left with some tension. The shape of the arms and the spring, and the initial tension, ensure a stable connection of the assembly. And they allow the operating movements: opening, closing and providing a clamping force.
LEGO pieces.
Lego pieces of all varieties form a universal system that is a typical case of form-fit closure. Despite the variation in design and purposes of individual pieces over the years, each piece remains compatible in some way with existing pieces. Lego bricks from 1958 still fit together with those made today, and Lego sets for toddlers are compatible with those made for teenagers. Six 2×4 stud bricks can be combined in 915,103,765 ways.
Dry stone.
Dry stone constructions, whether natural or worked, base their stability on the relative shape of the stones and their arrangement as a whole. Considering gravity and friction, they constitute a traditional solution and an example of form-fit closure. In 2018, UNESCO inscribed this practice on the Representative List of the Intangible Cultural Heritage of Humanity, under the name "Knowledge and techniques of the art of building dry stone walls", affecting the territory of Croatia, Cyprus, France, Greece, Italy, Slovenia, Spain and Switzerland.
Primitive weapons.
Javelin launchers have a tooth or protuberance for the base of the projectile to rest on. The temporary connection between the propellant and the projectile is a form-fitting closure.
Transmatic Transmission.
In 1987, a continuously variable transmission based on a metal belt (multi-piece) was marketed. The design and manufacture were also by Van Doorne. The operating system was similar to that of the Variomatic . But the new metal belts worked by compression, unlike the rubber ones, which transmitted the force by traction. The multi-piece belt consisted of a multitude of small steel pieces mounted between two flexible hardened steel belts. Each small piece was guided by a strap on each side, forming a form-fit closure.
Puzzle.
There are several three-dimensional puzzles based on form-fit closures.