For practical reasons, examples are drawn from only two Germanic languages: English and German
The general conclusions drawn on the basis of the comparison of English and German also apply, by extrapolation, to the other Germanic languages
Mutual intelligibility
Varieties which are mutually intelligible are dialects of the same language; varieties which are not mutually intelligible are languages
English and German are not mutually intelligible, hence, they are different languages
Structural similarity
Varieties which are structurally similar are dialects of the same language; varieties which are structurally different are languages
Linguistic criteria
Mutual intelligibility
Structural similarity
Non-linguistic (or extra-linguistic) criteria
Long tradition of use in literature
Prestige
Different national identities
English and German should be regarded as being different languages
English and German exhibit significant differences at all levels – i.e. phonology, morphology, syntax and the vocabulary
English and German: long tradition of use in literature
English and German: enjoy prestige
Speakers of English and German: different national identities, i.e. they identify themselves with different countries, different nations, etc.
English and German are different languages, rather than different dialects of the same language
Basic vocabulary
Includes a.o. names of basic body parts; names of basic degrees of kinship; personal pronouns; basic adjectives; names of basic colours; cardinal numbers from 1 to 10; basic verbs
Non-basic vocabulary
It consists of cultural, technical, scientific terms
The basic vocabulary: less likely to be borrowed vs. the non-basic vocabulary: frequently borrowed
English and German: lexical items which are similar in form and meaning are part of the basic vocabulary
Ablaut or apophony
Change of the root vowel to obtain various forms in the paradigm of a word
Suppletion
Use of different roots for the different forms in the paradigm of a word
English and German display significant similarities, in the basic vocabulary, in the regular morphology, in the irregular morphology, and even in suppletive forms
Comparative linguistics must provide answers to both these questions