An empty morph is an interfix if it fulfill all of the following three conditions [Dressler, W., 1985f, Zur Wertung der Interfixe in einer semiotischen Theorie der Natürlichen Morphologie, “Wiener Slavistischer Almanach”]. (1) It must be an affix which separates stem and the first (meaningful) suffix. (2) It must have no meaning whatsoever, and it must not be even a systematic co-signal of the following suffix category. (3) It cannot be described as being inserted by an MPR (within a theory that distinguishes MPRs from allomorphic MRs).
- Dressler (2004), Pag. 187