text: A typically Gascon feature that may arise from this substrate is the change from "f" to "h". Where a word originally began with  in Latin, such as festa 'party/feast', this sound was weakened to aspirated  and then, in some areas, lost altogether; according to the substrate theory, this is due to the Basque dialects' lack of an equivalent  phoneme, causing Gascon hèsta  or . A similar change took place in Spanish. Thus, Latin facere gives Spanish hacer () (or, in some parts of southwestern Andalusia, ). Another phonological effect resulting from the Basque substrate may have been Gascon's reluctance to pronounce a  at the beginning of words, resolved by means of a prothetical vowel.
  Although some linguists deny the plausibility of the Basque substrate theory, it is widely assumed that Basque, the "Circumpyrenean" language (as put by Basque linguist Alfonso Irigoyen and defended by Koldo Mitxelena, 1982), is the underlying language spreading around the Pyrenees onto the banks of the Garonne River, maybe as far east as the Mediterranean in Roman times (niska cited by Joan Coromines as the name of each nymph taking care of the Roman spa Arles de Tech in Roussillon, etc.). Basque gradually eroded across Gascony in the High Middle Ages (Basques from the Val d'Aran cited still circa 1000), with vulgar Latin and Basque interacting and mingling, but eventually with the former replacing the latter north of the east and middle Pyrenees and developing into Gascon.