The following features are characteristic of Insular Celtic, a subfamily of Indo-European spoken in Great Britain and Ireland. They are also characteristic of the so-called Hamito-Semitic languages of North Africa, a subfamily of Afro-Asiatic. Not every Celtic or H-S language has all of the features, but they are fairly typical of the family as a whole, especially in their most archaic variants (e.g. Old Irish).
- Conjugated prepositions: [Prep + Pronominal Obj] is a single word.
- Word order: VSO, N - Modifier, Prepositions
- Relative clause linker: invariant particle, not relative pronoun
- Relative clause technique (oblique): copying, not gapping, e.g. "the bed that [ I slept in it ]".
- Special form of the verb peculiar to relative clauses.
- Polypersonal verb (subject and object both marked).
- Infixing/suffixing alternation: Object marker is infixed to the verb if there is a preverb, suffixed otherwise.
- Definite article in genitive embeddings may occur only on the embedded noun: "house [the-man]" = the man's house.
- Nonconcord of verb with full-NP subject: verb can fail to agree with the subject, depending on word order.
- Verbal Noun (VN: object in genitive), not Infinitive (object in same case as with finite verb).
- Predicative particle: in copular or nominal sentences, the predicate is marked with a participle homophonous to a "local" preposition: "He (is) in a farmer" = He is a farmer.
- Prepositional periphrastic: BE + Prep + VN, e.g. "He is at singing".
- DO periphrastic: DO + VN, e.g. "He does singing".
- Notional adverbial clause expressed as "and" + finite clause.
- Nonfinite forms usable instead of finite main-clause verb.
- Word-initial phonological change, expressing a variety of syntactic functions.
- Idiomatic use of kin terms in genitive constructions, e.g. "son of sending" = messenger, "son of land" = wolf.
(Source: Orin Gensler, A Typological Evaluation of Celtic/Hamito-Semitic Parallels, Berkeley, 1993.)
Note that this distinctive feature-complex is not typical of other Indo-European languages, and does not seem to be present in Continental Celtic. This has led to the supposition of a substrate/sprachbund situation in the ancient British isles: the language spoken by the inhabitants before the arrival of Celtic was in heavy contact with Afro-Asiatic speakers and shared these distinctive features, in the usual sprachbundy way; this pre-Celtic language influenced the Celtic language imposed by the Indo-European conquerors in the usual substratal way.
No other language groups of the world seem to share this set of 17 features to any statistically significant degree; this is unlikely to be the working-out of typological universals.