In the first century BCE, Gaulish is found over a very large area.

Modern France, the west bank of the Rhine, the Low countries up to the Rhine mouth, and the Swiss Jura are referred to as Transalpine Gaul. The Po basin up to the Alpine watershed is referred to as Cisalpine Gaul.

Close contact with the British Isles will have ensured that British as spoken in the south and southwest of England was close to Gaulish.

Galatian, a Continental Celtic language probably close to Gaulish, was spoken in Asia Minor and Thrace as a result of the invasion and settlement of these areas by the Gauls in the third century BCE.

Caesar noted that in transalpine Gaul three different dialects were in use and that the three regions of Gaul were distinguishable linguistically.

Just enough has survived of Gaulish to indicate that this could have been so. He calls the inhabitants of the three regions the Belgae, Aquitani and Celtae and informs us that the latter were called Galli by the Latins.

An early form of Basque co-existed with Gaulish in Aquitane.