Because of typological implications of Sturtevant's law, the distinction between the two series is commonly regarded as one of voice.
However, there is still disagreement over the subject among scholars, some of whom view both series as if they were differentiated by length, which a literal interpretation of the cuneiform orthography would suggest.
Supporters of a length distinction usually point to the fact that Akkadian, the language from which the Hittites borrowed the cuneiform script, had voicing.
If the distinction were one of voice, agreement between the stops should be expected since the velar and the alveolar plosives are known to be adjacent since the "u" in that word does not stand for a vowel but instead represents labialization.
[1] All resonants in Hittite coincide with their respective etymological pronunciations in Proto-Indo-European, which makes it unlikely that they were pronounced differently.
[4] Resonants in Hittite could be geminate or simple,[5] but that distinction was not inherited from Proto-Indo-European but is instead often believed to have been caused by assimilation.
That is known since "t"-stem nouns have a nominative ending in "z" and some verbal desinences descended from a prehistorical sequence "*ti".
[9][failed verification] A secondary source of the sign "z" is an early Indo-European dissimilation that occurred between two adjacent dentals, which consisted in the insertion of "*s" between them (e-ez-du - [ɛ́ːt͡stu] < "*h₁éd-tu").
Some advocates of a voice/voiceless series propose a voiced counterpart, which is rather controversial, and claim that whenever "z" was geminate, it represented [t͡s] and that when it was simple, it was pronounced [d͡z].
[12] Although the exact place of articulation of the Hittite phoneme written with signs having an "š" cannot be determined with absolute certainty, there are various arguments for assuming it to have been an alveolar sibilant.
It has also been noted that Hittite royal names containing an "š" are written in Egyptian Hieroglyphs with the sign that is conventionally transcribed as "s".
[19] Furthermore, Ugaritic borrowings from Hittite commonly transcribe "ḫ" as "ġ", which stands for a voiced velar fricative (e.g., dġṯ < duḫḫuiš, tdġl < mTudḫaliya, trġnds< URUTarḫuntašša).
[27] Similarly, that evidence precludes the possibility of "ḫ" being a pharyngeal fricative, which usually triggers the fronting, rather than retraction, of vowels (e.g., Proto-Semitic *ḥarāṯum > Akkadian erēšum).
That view is also strengthened by the first-person plural present form "a-ku-e-ni - [agwɛ́ni]", instead of the expected *a-ku-me-ni, as in Hittite true verbal u-stems.
It has also been noted that it can be written as "e-uk-zi - [ɛ́gʷt͡si]", which has been pointed out as an argument for assuming labialization as well, in which case the rounding happened with at the same time as the plosive instead of following it as a semivowel.
[42] It received mainstream support only when Kloekhorst published in 2008 a detailed analysis of the distribution of both signs and found significant evidence for their being contrastive.