The early Anglo-Frisian and Old Saxon speech communities lived close enough together to form a linguistic crossroads which is why they share some of the traits otherwise only typical of Anglo-Frisian languages.
To a lesser extent they occur in three neighbouring groups of Bantu languages—which borrowed them, directly or indirectly, from Khoisan.
Roger Blench notes that the Talodi and Heiban families have the noun-class systems characteristic of the Atlantic–Congo core of Niger–Congo, but that the two Katla languages have no trace of ever having had such a system, whereas the Kadu languages and some of the Rashad languages appear to have acquired noun classes as part of a Sprachbund rather than having inherited them.
Shared syntactic features include classifiers, object–verb order and topic–comment structure, though in each case there are exceptions in branches of one or more families.
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In a 1904 paper, Jan Baudouin de Courtenay emphasised the need to distinguish between language similarities arising from a genetic relationship and those arising from convergence due to language contact.
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The term Sprachbund, a calque of the Russian term языковой союз (yazykovoy soyuz; "language union"), was introduced by Nikolai Trubetzkoy in an article in 1923.
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Emeneau specified the tools to establish that language and culture had fused for centuries on the Indian soil to produce an integrated mosaic of structural convergence of four distinct language families: Indo-Aryan, Dravidian, Munda and Tibeto-Burman.
Roger Blench notes that the Talodi and Heiban families have the noun-class systems characteristic of the Atlantic–Congo core of Niger–Congo, but that the Katla languages (another putative branch of Kordofanian) have no trace of ever having had such a system, whereas the Kadu languages and some of the Rashad languages appear to have acquired noun classes as part of a Sprachbund, rather than having inherited them.
Along with Lyle Campbell and Thomas Smith-Stark, Kaufman carried out research published in Language (1986) which led to the recognition of Mesoamerica as a linguistic area.
Dixon (2002) considers them to be language isolates with no demonstrable relationship other than that of a Sprachbund.
A difference in reported material origin with Chukotko-Koryak languages in declensional and conjugational paradigms is the result of convergent development under conditions of a Chukotko-Kamchatkan Sprachbund.