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In page Celts:

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Diodorus Siculus and Strabo both suggest that the heartland of the people they call Celts was in southern Gaul. The former says that the Gauls were to the north of the Celts, but that the Romans referred to both as Gauls (linguistically the Gauls were certainly Celts). Before the discoveries at Hallstatt and La Tène, it was generally considered that the Celtic heartland was southern Gaul, see Encyclopædia Britannica for 1813.[citation needed]