This paper illustrates how semantic properties of linguistic structures and forms highlighted by synchronic work can be used diachronically, in particular in etymological studies. With this aim, we focus on two particular classes of part nouns from Basque for which it appears that, while having opposite properties, they make up a real linguistic continuum. Then, we use these synchronic data in order to explain -through the concept of "foot/leg"- the etymology of various locative markers (spatial and sometimes spatio-temporal) calling for orientational notions.