Related languages have a number of words which are similar to one another. In the branch of linguistics known as historical linguistics, the proto-form is defined as the word from which similar words were derived. Determining the proto-form often involves a great deal of speculation because of the lack of written records. However, common phonological processes such as weakening (lenition), strengthening (fortition), assimilation, syllable structure and neutralization are useful for determining the proto-form of similar words.
Danish, Norwegian and Swedish are three similar languages which are very useful for determining proto-forms. Here is a list of ten words in these languages which are rather similar:
Danish efter Norwegian etter Swedish efter (after)
Danish kage Norwegian kake Swedish kaka (cake)
Danish ti Norwegian ti Swedish tio (ten)
Danish syg Norwegian syk Swedish sjuk (sick)
Danish hvad Norwegian hva Swedish vad (what)
Danish skib Norwegian skip Swedish skepp (ship)
Danish gade Norwegian gate Swedish gata (street)
Danish mave Norwegian mage Swedish mage (stomach)
Danish hvid Norwegian hvit Swedish vit (white)
Danish peber Norwegian pepper Swedish peppar (pepper)
The proto-form from which similar words are derived is marked with an asterisk as follows: *.
An arrow is used to show the resulting form. For example, to show that the French word "quatre" is derived from the Latin "quattuor", we can write it in this manner: *quattuor ---> quatre.
Now we can consider the Danish, Norwegian and Swedish words. The word "after" is the same in Danish and Swedish. On this basis alone, we may follow the rule of majority rules and choose the one present in two out of three languages. However, we also have natural development to suggest that the Norwegian form "etter" is the result of an assimilation process. Thus we can determine the protoform. It is *efter.
The word "cake" is different in all three languages. We can suggest that the proto-form is *kaka.
Vowel weakening changes the word-final "a" to "e" and voicing changes the "k" to "g".
The proto-form of "ten" is *tio. Word-final vowel deletion derives "ti".
The word "sick" is different in all three languages. The proto-form is *syk. Palatalization changes *syk to "sjyk" in Swedish and vowel retraction changes "sjyk" to "sjuk". In Danish, the plural adjective "syge" exists. Thus we can postulate intervocalic voicing to change the "k" to "g" and then word-final vowel deletion.
The proto-form of "what" is *hvad. Word-final consonant deletion derives the Norwegian word "hva" and consonant cluster simplication derives the Swedish word "vad".
The proto-form of "ship" is *skip. Voicing derives the Danish plural "skibe" (ships) and the singular "skib". As for Swedish, a vowel change derives "e" and the double "p" merely indicates that the vowel is short.
The proto-form of "street" is *gata. Word-final vowel weakening derives the Norwegian word "gate" and the additional rule of intervocalic voicing derives the Danish word "gade".
The proto-form of "stomach" is *mage. Majority rules applies because this is the form in two out of three languages. Natural development is also in evidence because the change from "g" to "v", from a plosive to a fricative, is an example of weakening.
The proto-form of "white" is *hvit. Danish has the adjective plural "hvide". Thus, "hvid" is derived as a result of intervocalic voicing and apocope, word-final vowel deletion. The Swedish word "vit" is the result of consonant cluster simplification.
The proto-form of "pepper" is *peppar. The Norwegian word "pepper" is the result of vowel weakening. The Danish word "peber" is the result of voicing and if the word "pebber" ever existed, we can apply degemination to derive "peber".
The proto-form is not always an existing form of a language. For example, the word "week" is "uge" in Danish, "uke" in Norwegian and "vecka" in Swedish. It appears that the proto-form is neither of these forms. By considering other languages such as English, Dutch and German, we can begin to determine the proto-form. English and Dutch have "week" and German has "Woche" for "week". It thus appears that the initial segment is a "w". It may be that the proto-form of "week" in Danish, Norwegian and Swedish is *weka. Strenghtening can change *weka to "ueka" in Danish and Norwegian. Vowel deletion changes "ueka" to "uka". Voicing then derives "uga" in Danish and word-final vowel weakening derives "uge". In Norwegian, "uka" changes to "uke" as a result of word-final vowel weakening. In Swedish, *weka changes to "veka" as a result of a phonological process which changes the labiovelar glide to a voiced labiodental fricative. Then gemination, a process needed to maintain the short "e", derives the form "vecka". However, this analysis may not be accurate. It involves a considerable amount of speculation, but this is what historical linguists do.
Languages which are closely-related to one another such as Danish, Norwegian and Swedish are very good for determining proto-forms. With less similar languages, the process of finding the proto-form is considerably more difficult.