In linguistics, prouctivity is the degree to which native speakers use a particular grammatical process, especially in word formation. Since use to produce novel (new, non-established) structures is the clearest proof of usage of a grammatical process, the evidence most often appealed to as establishing productivity is the appearance of novel forms of the type the process leads one to expect, and many people would limit the definition offered above to exclude use of a grammatical process that does not result in a novel structure. Thus in practice, and, for many, in theory, productivity is the degree to which native speakers use a particular grammatical process for the formation of novel structures. A productive grammatical process defines an open class, one which admits new words or forms. Non-productive grammatical processes may be seen as operative within closed classes, but only previously formed and learned structures show the results of those processes.