The traditional interpretation of the data is that these two subgroups of the Indo European language family are generally more closely related to each other than to the other Indo European languages. This is usually taken to imply that they are descended from a common ancestor, a phylogenetic Proto-Italo-Celtic which can be partly reconstructed by the comparative method.
Based on features of Italic and Celtic languages, the following kind of endings have been reconstructed for Italo-Celtic:
|Secondary active||Transitive middle||Passive||Intransitive middle|
|Sg.3||-t||-to||-toro||-o, -oro, -tro|
- Kortlandt, Frederik. Italo-Celtic origins and prehistoric development of the Irish language. New York. 2007.