Sometimes the general structure suggests a zero element. For example, in Totonac [...] the third person singular is never indicated overtly, i.e. it has no obvious form. Structurally, this is a type of significant absence; it is not, however, an allomorphic zero, but, rather, a morphemic zero. That is to say, this significant absence does not occur in a series of allomorphs, but in a series of morphemes.
- Nida (1949), a pag.46