Skou languages are unusual among Papuan languages for being tonal; all Skou languages possess contrastive tone. Vanimo, for example, has three tones, high, mid, low. Example minimal sets illustrating tonal contrasts in various Skou languages:
Barupu: eH ‘tooth’, eL ‘garden’, eHL ‘mosquito’, eHLH ‘write’
Wutung: hoH ‘roof thatch made from sago palm fronds’, hoL ‘star’, hoHL ‘grease’
Skou: taH ‘grass’, taL ‘hair’, taHL ‘arrow’
Lakes Plain languages, spoken in a discontiguous area to the southwest, are also tonal. Because of the apparent phonological similarities and sharing of stable basic words such as ‘louse’, Foley speculates the potential likelihood of a distant relationship shared between the Skou and Lakes Plain families, but no formal proposals linking the two families have been made due to insufficient evidence. Additionally according to Foley, based on some lexical and phonological similarities, the Keuw language may also possibly share a deep relationship with the Lakes Plain languages. Like the Lakes Plain languages, Keuw also possesses constrative tone. Lepki, Kaure, and Kembra, spoken in mountainous inland regions of the Indonesia-PNG border to the southwest of the Skou-speaking area, are also tonal.
Skou languages were first linked by G. Frederici in 1912. In 1941, K.H. Thomas expanded the family to its current extent. The Sko family is not accepted by Søren Wichmann, who splits it into two separate groups. Donohue and Donohue and Crowther listNouri as a mixed language having features of both the Piore River and Serra Hills subgroups.
provides the following classification. Foley's Inner Sko corresponds to Donohue's Western Skou.
Miller (2017)
The Piore River branch was renamed Lagoon in Miller. The older names of the Piore River languages were from village names; Miller has since renamed them as Bauni, Uni, Bouni, and Bobe, though they are not all distinct languages. Lagoon
Bauni
Uni
Bouni
Bobe
Usher (2020)
Usher groups the languages as follows, with each node being a reconstructable clade, and giving the family a geographic label rather than naming it after a single language. Usher retains only the two Piore River languages of Donohue but splits off Sumararu as a distinct language.
The pronouns Ross reconstructs for proto-Skou are, The Skou languages also have a dual, with a distinction between inclusive and exclusive we, but the forms are not reconstructable for the proto-language. Pronouns in individual Skou languages: