A code wheel is a type of copy protection used on older computer games, often those published in the late 1980s and early 1990s.
This system was popular because it allowed the actual media to be backed up and replaced freely while retaining security, but with the increased availability of photocopiers, those wishing to distribute the games simply started copying their manuals as well to defeat the measure.
Once a suitable formula was found, code wheels actually made the process of copying easier since the amount of information they could contain was low compared to a manual of potentially unlimited size.
A code wheel itself is a physical object consisting of several circular sheets of paper or card of different sizes, fastened in the center, creating a set of concentric circles.
This type of codewheel was used for a large number of games, such as Neuromancer, and Cybercon 3 (which used a code wheel printed on carbon paper).