By comparing variants of the same word in modern dialects and the changes that Latin loanwords had undergone, he deduced the ancestral forms and the rules for historical sound changes.
His groundbreaking work, which culminated with the publication of his book Fonética histórica vasca (1961) (a revised version of his doctoral thesis of 1959), was carried out mostly before the Aquitanian inscriptions were found, but they fully backed up Mitxelena's proposed Proto-Basque forms.
[3] Since then, a number of other prominent linguists, such as Larry Trask, Alfonso Irigoien, Henri Gavel and most recently Joseba Lakarra, Joaquín Gorrotxategi and Ricardo Gómez, have made further contributions to the field.
[4] Onomastic attestations of the Aquitanian language, which is only known from the names of places, persons and deities in inscriptions from the first centuries CE, closely match the reconstructed form of Proto-Basque.
Joseba Lakarra proposes that in Pre-Proto-Basque there was extensive reduplication[15] and that later, certain initial consonants were deleted, leaving the VCV pattern of Proto-Basque: