A debate among the MULTEXT partners generated some possible definitions of common category on which it is worth reflecting:
We could define common categories as
Common categories: Categories whose members satisfy the same criteria and tests.
This crucially implies a clear definition of criteria for the recognition of their members (see the guidelines produced in the validation phase). In the absence of such explicit criteria, we can empirically recognise common categories which are relevant in the morphosyntactic descriptions of a number of European languages. These common categories are usable, are actually used in the largest lexicons and corpora, and have in general the same meaning in the different languages, even though the property of commonality holds more for open classes and poses more problems for function words or closed classes.
In fact, with the above caveat in mind, it was found from the analysis of the schemes that many of the commonalities here proposed are relevant for many languages. Different schools and traditions of languages can agree on such a simple set of features.