Aquae Sextiae → Aix

The ancient name for Aix-en-Provence appears in Pliny the Elder’s encyclopedia, where the Roman scholar, who died during the eruption of Vesuvius in 79 CE, lists it with other Roman towns and colonies : oppida Latina : Aquae Sextiae Salluviorum…The Salluvii were a Liguro-Celtic tribe of the region ; they were one of the peoples against whom the Roman consul Sextius Calvinus (mid 2^nd CE BCE) fought, and for which he claimed a triumph back in Rome. In 122 BCE, he established in the area of the battle a town, giving the springs his name, Aquae Sextiae. How does one go from the toponym Aquae Sextiae to Aix? The answer : the Roman locative case form, an ending (in –īs)  which, when added to a noun gives it the meaning « at the… », so Aquis Sextiis, « at Sextius’ Springs ». This form had become the city’s name by the 4th century CE, when it is referred to as Aquis Sestis (Nègre vol. 1 §5016). From Aquis, to Aix, seems clear enough: if you pronounce Aquis quickly, you can hear it for yourself.

Reference: Nègre, E. 1990. Toponymie Générale de la France. Vol. 1er: Formations Préceltiques, Celtiques,  Romaines. Geneva.