Some, but not all, scholars of those languages argue for their genetic relationship based on putative systematic sound correspondences, while the consensus among general linguists is that this hypothesis is at best speculative and by no means proven.
The similarity between the Khoe and !Xóõ forms for 'drink' and 'laugh' hints at possible sound correspondences between the vowels and the consonants, but this similarity fails to extend to other words in the two groups.
The Uralic and Altaic language families were once believed to form a superfamily, but reliable sound correspondences have not been demonstrated, and the numerous similarities between the two are now attributed to areal influences.
The theories of the Neogrammarians, a group of German historical linguists and classical scholars who first gained prominence in the 1870s, were especially important because of the rigorous manner in which they formulated sound correspondences in the Indo-European languages.
A genetic relationship between the Northeast and Northwest languages seems probable, but the absence of regular sound correspondences between North and South Caucasian languages strongly suggests that the two northern divisions form a family separate from the southern group.
It rests on the assumption that sound correspondences in related words and morphological units, as well as structural similarities on all levels (phonology, morphology, syntax), can be explained in terms of a reconstructed common language, or protolanguage.
Conventional methods of linguistic comparison applied between the main groups of the Khoisan languages have failed to yield regular sound correspondences, which would allow common roots to be reconstructed; and shared innovations in grammatical structure, which are regarded as the best source of evidence for postulating linguistic relationships are, frustratingly, absent.
As mentioned above, many scholars who work on Turkic, Mongolian, and Manchu-Tungus languages today consider a genetic relationship between those languages to have been proved and hence regard the Altaic group as a language family, basing that conclusion not only on similarities in vocabulary and language structure but on well-established systematic sound correspondences as well.
On the basis of proposed sound correspondences, the hypothesis of a genetic relationship between Altaic and Korean is regarded by some scholars as proved, but, while most scholars view the relationship as worthy of further investigation, it has not as yet won universal acceptance.
Overall historical-comparative work in the strict sense, using Neogrammarian notions of regular sound correspondences between cognate forms in related languages and notions of shared innovations in order to arrive at proper subclassifications (as developed in the comparative study of Indo-European languages), is still lacking for the family as a whole.
