Proto-Basque is the reconstructed ancestor of the historical Basque dialects, presumably spoken in the Late Iron Age and whose main features were worked out by the Vascologist Koldo Mitxelena in his magna opus Fonética Histórica Vasca1. The consonantal system of Mitxelena's Proto-Basque didn't had a voiced/voiceless contrast but a fortis (tense)/lenis (lax) one. This was only relevant in medial position (between vowels), because at word-initial only lenis phonemes might appear.
With regard to stops, the French linguist André Martinet (Économie des changements phonétiques, 1955) posited an older system modelled after Danish, in which plosives could be realized either as voiceless aspirated (fortis) or mild voiceless (lenis) at word-initial and as mild voiceless (fortis) and approximants (lenis) between vowels. In Mitxelena's Proto-Basque, the aspirated plosives would have evolved to [h], which had no phonemic value.
I call this process Martinet's Law, and for comparative purposes we should differentiate between Early Proto-Basque (before Martinet's Law) and Late Proto-Basque (after Martinet's Law), sometimes called "canonical" or "Mitxelena's" as opposed to Bengtson's.
In the Aquitanian inscriptions we can found forms with initial h- like HALSCO or HAVTENN2 vs. others with t- like TALSCO (Iberian talsku) or TAVTINN (Iberian tautin) and different geographical distribution. This isogloss is a particular case of Martinet's Law, which separates Proto-Basque from other linguistic varieties like Iberian.
There are also Basque words like k(h)ako 'hook'3 < IE *ko(n)g- 'peg, hook, claw' < Paleo-Eurasian *ɣoŋɣV 'peg, nail' or tara 'young branch' < IE *dhal- 'sprout' which don't follow Martinet's Law and constitute what I call non-standard Basque4. They are late loanwords from those linguistic varieties, whose speakers were stigmatizated people like highlanders or nomadic shepherds, a fact which lead to their ultimate extinction in the High Middle Ages (there's evidence in NW Catalonia's toponymy of a Bascoid language spoken there until aprox. 1000 AD).
Unlike his original proposal, Martinet's Law also can occur between vowels. For example, Basque zahar 'old' < EPBasq. *sakhaŕ5 vs. Iberian sakaŕ (Starostin's PNC *tʃ’HǝqwV 'big').
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1 For a summary of Mitxelena's work, see Trask's The history of Basque (1997).
1 For a summary of Mitxelena's work, see Trask's The history of Basque (1997).
2 Compare Basque hauta 'election, excellent' < Celtic *toutā- 'people'.
3 In the Salazarian dialect there's an isolated form kaiku 'aquiline' found part of the compound sudur-kaiku 'aquiline nose'.
3 In the Salazarian dialect there's an isolated form kaiku 'aquiline' found part of the compound sudur-kaiku 'aquiline nose'.
4 It's a pity Bengtson didn't recognize this as a genuine phenomenon.
5 Basque /z/ represents a lamino-alveolar sibilant [s].
