Why some people say git and some say gut: The mechanism of Yiddish dialect differentiation

  The sounds of a language can change in different ways. The most commonly described change is a sound shift. A classic example from the history of English is that a long U sound inherited from ancient Germanic became a diphthong the we find in words like 'cow.' Another way that sounds can change is when the words they are found in are borrowed from one language to another. The speakers of the new language adapt the pronunciation of the word to fit in with their own habits of speaking. This second process is what underlies most of the differences that we find in pronunciation between Yiddish dialects.
  To illustrate this we can take an example from the Yiddish dialects historically spoken in Ukraine, Poland, and Belarus and the Baltic countries. The word for 'good' is pronounced either as git or gut. To explain this fact we have to explain two things. One is how the difference came about and the other is why which pronunciation is historically preferred in which areas. In what follows, I will set questions of geographical distribution aside and deal with the origin of the difference. I will also not delve into variations in the pronunciation of I vowel among people who say git as these are not relevant to this particular problem.
  To begin with we are looking at a very big pattern. The alternation of u and i is found in words of Germanic origin like git and gut as well as words of Slavic origin like the word for 'beetle' that can be zhik or zhuk and words of Semitic origin like the word for 'crazy' that can meshige or meshuge. In all cases a speaker who uses a u or i in one of these words uses the same vowel consistently in the other, When linguists encounter a pattern like this, the first explanation they think of is that there was a sound shift. These words originally had one of these sounds, perhaps an I sound, perhaps an U and then some speakers regularly changed their pronunciation from one form to another. At first historical facts seem to support this. All these words once had an U in some forms of Germanic, Slavic and Semitic.
  A closer look at the facts allows us to regard such a sound shift unlikely. A sound shift has a limited duration. During the time it is happening all the words with appropriate sound undergo the shift. Then the shift comes to an end. After the shift has run its course new words borrowed into the language no longer undergo the shift. If they have the sound affected by the shift, say I or U, they keep it. Since Germanic, Slavic and Semitic words were borrowed into Yiddish over a long period extending into recent times we would expect, if there were a shift from U to I, that the shift would have come to an end at some time and words borrowed later that had U would keep it even among speakers who say git.
  We do not find this. People who say git use an I sound consistently in words where other people say gut even in words that came into the Yiddish language in historically recent times. Another explanation is required. This would be that the changes in pronunciation are the result of borrowing. It was borrowing of a specific kind and to see how this happened we have to take a closer look at the structure of Yiddish.
  Max Weinreich described Yiddish as a fusion language. A fusion language consists of components of different etymological origins. Each component borrows material from a specific class of languages called its determinant. Yiddish has a Germanic component with a Germanic determinant, as well as a Slavic and a Semitic component.
  While all elements of Yiddish share common linguistic structures, each component also has its own structural features. Plurals, for example, are generally constructed differently in nouns of the Germanic, Slavic and Semitic components.
  Each component also has its own ways of adapting words from its determinant into the Yiddish language. Typically, for example, borrowed nouns are assigned new plurals. The plural of noun borrowed from Germanic often has a different ending than it had in its Germanic determinant. This is often true in the case of nouns from the Slavic component and, occasionally,  in nouns from the Semitic component as well. This fitting words from determinants into components is the characteristically Yiddish form of linguistic borrowing.
  Ways of adapting words from the determinants to the components can vary somewhat within Yiddish. For example, words from the Germanic components can be assigned different plurals in Lithuania and Belarus than they might be assigned in Poland or Ukraine.
  The same principles apply to pronunciation. The Germanic, Slavic and Semitic components have characteristic ways of adjusting the pronunciations of words from their determinants to fit in with Yiddish patterns. As with the case of plurals these ways of adapting pronunciation can vary somewhat within the language.
  Now we can see how the git and gut pronunciations could have emerged. Languages vary in the kinds of U sounds they can have. Some like French and German have two kinds. One kind is usually spelled 'ou' in French and 'u' in German. In phonetic notation it is written /u/. The other kind is usually 'u' in French 'u' with an umlaut in German. It is written /y/. English has only one kind of u. In standard English it is /u/ but in dialects like the one spoken in Philadelphia it is /y/.
  It is possible to infer from old manuscripts that Yiddish in medieval Germany was one of the languages that distinguish /u/ and /y/. Bu those Yiddish speakers who migrated into eastern Europe lost this distinction. Linguistic distinctions are often lost in migrations.
  Once the distinction was lost, Yiddish speakers had to opt either for /u/ or /y/ as their one preferred pronunciation. Some communities opted for /u/ and others for /y/This had an effect on the way words from the Germanic, Slavic and Semitic determinants were incorporated into the components. Communities that used /u/ assigned /u/ to words with U sounds in the determinants while those with /y/ assigned /y/ to the same words.
  In eastern Europe Jews found themselves in different linguistic environments. Hungarian had both /u/ and /y/ sounds while Polish, Ukrainian and Belarusian had only /u/. Yiddish speakers in the territory of modern Hungary and adjacent Burgenland, Austria and western Slovakia arrived with the  /y/ variant and kept it. In the Slavic lands some settlers founded communities with the /u/ variant. They maintained it in their new homes. Others founded communities with the /y/ variant but these lost it under the influence of surrounding languages. They shifted from /y/ to the sounds closest to it which were I sounds.
  The shift from /y/ to I sounds changed the way words from the determinants were incorporated into the components. While previously these communities had incorporated words with U sounds from the determinants as having /y/ they now incorporated them with I sounds. As a result Yiddish speakers in the Slavic lands either said git or gut. (An exception was the town of Bransk, Poland which still has /y/.)
  This is an illustration of the power of the patterns that govern the way words from the determinants are incorporated into the components. As long as Yiddish remained a fusion language these patterns were strong enough so that words with U were taken into the components as having I vowels. The principle mechanisms by which Yiddish dialects arose were not sound shifts but rather variations in way elements from determinants were adapted to fit into components.