First, he looks at a number of morphological and phonetic isoglosses:
1. The 3rd person singular preterite of CC verbs: yərra vs. yərru. Following Kossmann (2001), we now know that these are actually CC+glottal stop, so the data exemplifies two different sound changes: the relatively trivial *-aʔ > -a, and and the more surprising *-aʔ > o > u. The former is the commonest outcome; the latter is exemplified by: Ait Seghrouchen, Figuig, Beni Snous, Bissa, Timimoun, Mzab, Ouargla, Nefusa. (Ghadames still has o).
2. The proximal demonstrative suffix: -a vs. -u. Again, -a is the default, but -u appears in the same set of varieties as seen in 1, plus one more: Iznasen.
3. The 3rd person singular aorist of CCV verbs: ad yəbḍu vs. ad yəbḍa. Here, -u is the default, and is closer to the original, while -a has spread from the preterite. This applies to the same set of varieties as 2 (excluding Nefusi), plus several more: Rif, Metmata, Chaoui, Jerba.
4. Initial vowel dropping: a- vs. 0-. A number of *(t)a-CV-initial nouns drop the original vowel of the prefix in the same set of varieties as 3, plus Nefusi, Chenoua, and Siwa.
5. Velar softening: in many varieties, in many words, what would elsewhere be k/kk/g/gg corresponds to c/čč/j/ǧǧ. The latter outcome is observed in the same set of varieties as 4, minus Nefusi.
6. Final *-əv: this is retained as such in Ghadames and Awjila, and as constrative length in Zenaga. Otherwise, it becomes -u in most varieties, but -i in the same varieties as listed in 4, plus El-Fogaha (with a few question marks where the author had insufficient data). Cf. Kossmann (1995).
All of 1-6 pick out Zenati varieties, but the exact set differs: 1-2 pick out a core Zenati consisting almost entirely of northern Saharan varieties, while 3-6 pick out a broader Zenati including the semi-arid mountainous lands stretching from the Rif to southern Tunisia, and vary in their inclusion of varieties further east (Nefusi, El-Fogaha, Siwi). Chaker (1972) cites 1-2 and 5 as possibly justifying a Zenati subgrouping, while Kossmann (1999) defines Zenati in terms of 3, 4, and one other morphological innovation, and then cites 5 and 6 as common phonological innovations.
7. Negative intensive theme: retention/loss. The negative intensive is retained in northwestern Morocco (Rif, Iznasen, Senhaja, Ait Seghrouchen, Figuig); in Bissa; in Tuareg and in the nearby oases of Mzab, Ouargla, and Ghadames; and in Jerba. Its loss everywhere else (according to his data, which should be re-checked) shows no prominent genetic patterning, and hence is probably relatively recent.
Then, he moves on to vocabulary, examining 11 lexical variables which I would summarise as follows:
Several forms appear specifically Zenati: irəḍ in the sense of "be dressed" (though it is more widespread in other senses), igur for "go", əɣs for "want", azəgrar for "long", anilti for "shepherd". Of these, El-Fogaha and Siwa share only əɣs for "want", whereas Nefusi shares all except "go in". adəf "go in" is Zenati-specific in the west, but more confusing in the east, being attested in Ghadames and (as an alternative to əggəz) in Air Tuareg.
Several forms appear specifically Tuareg: răgăz for "go", amaḍan for "shepherd", əggəz in the sense of "go in" (elsewhere "go down"), zəgrət (with the extra t) for "long".
One form unites southern/central Morocco with Kabyle: awtul "hare" (vs. pan-Berber a-yərẓiẓ.)
A couple of forms unite Libyan varieties with Tuareg, contrasting with Algerian and Moroccan varieties, in defiance of any plausible genetic classification, reminding us that a tree does not tell the whole story here:
- iziḍ "donkey" (Tuareg, Ghadames, Nefusi, Siwa, Awjila) vs. aɣyul (everywhere else except El-Fogaha)
- tufat/tifut/tafyi "tomorrow" (Tuareg, El-Fogaha, Siwa respectively) vs. azəkka (everywhere else except El-Fogaha, Awjila, and Zenaga)
Based somehow on all this, he proposes the following very odd tree:
- Group 1
- Senhaja, Middle Atlas, Shilha, Kabyle, Zenaga
- Group 4 ("Zenati")
- Ait Seghrouchen, Beni-Snous, Timimoun, Figuig, Bissa, Mzab, Ouargla
- Iznasen, Jerba
- Rif, Metmata, Aures
Apparently, to get this he operated by successively applying at each stage the criterion from his list that divided the data into the lowest number of groups possible, without attempting to distinguish innovations from retentions, much less judge the relative likelihood of independent innovation. The fact that even such a crude method was still able to produce a recognisable Zenati subgroup either says something about the robustness of this distinction or about the selection of features. What this data set actually tells us, bearing in mind that shared retentions have no implications for subgrouping and that Zenaga fails to participate in a number of innovations that otherwise seem pan-Berber or nearly pan-Berber, is something quite different:
- There is definitely a Zenati subgroup, as has been known at least since Destaing (1915), but its boundaries are a bit fuzzy. (If this reminds you of the situation of "Hilalian" g-dialects, that's probably not a coincidence.)
- Western Zenati:
- Core (mainly Northern Saharan): Ait Seghrouchen, Figuig, Beni Snous, Bissa, Timimoun, Mzab, Ouargla
- Transitional (the High Plateau and its edges): Rif, Metmata, Chaoui, Jerba
- Chenoua (north-central Algeria)
- Nefusi (northwestern Libya)
- Eastern Zenati (Libya/Egypt): El-Fogaha, Siwa
- Western Zenati:
- There is definitely a Tuareg subgroup, as has always been known: Ahaggar, Iwellemmeden, Air, Taneslemt.
- There just might be a subgroup combining Kabyle with Senhaja, Central Morocco and Shilha: they share the innovation *-əv > -u, and the word awtul "hare". The evidence for it is very weak, though, especially since *-əv > -u is also found in some Tuareg varieties.
The rest of the common features almost all look like shared retentions.