Besides common linguistic innovations, speakers of these languages shared cultural features and history.
The cultural aspects are commonality of art styles and worship of similar gods.
In Julius Caesar's time, the Atrebates held land on both sides of the English Channel.
It contrasts with the Insular Celtic hypothesis, which asserts that Goidelic and Brythonic underwent a period of common development and have shared innovations to the exclusion of Gaulish,[1] while the shared changes are either independent innovations that occurred separately in Brythonic and Gaulish or are due to language contact between the two groups.
These innovations are not shared with the Goidelic languages, which also called Q-Celtic in this model because of their preservation of Proto-Celtic */kʷ/' (often represented as qu in English).