The Zero one or infinity (ZOI) rule is a rule of thumb in software design originated by early computing pioneer Willem van der Poel. It suggests that arbitrary limits on the number of instances of a particular entity should not be allowed. Specifically, an entity should either be forbidden entirely, one should be allowed, or any number (presumably, to the limit of available storage) of them should be allowed. It should not be the software that puts a hard limit on the number of instances of the entity.
Examples of this rule may be found in file directories. The top-most directory has zero parent directories. Each subdirectory has exactly one parent directory. Each directory may contain an arbitrary number of files or subdirectories. An example of a violation of this rule is that some file systems impose a hard limit of 65,536 files in a directory.