I wondered about this today. I can give you an example of what I mean. Let's take some language which in reality doesn't exist, but of which we will assume that it exists in a parallell exactly same universe, the language 'Scotlèn', by which we have to take in mind that the development of the English language is the only thing which is different in this parallell universe. This language is the development of a form of spoken English in Scotland which is quite different from the development of Scottish in our own reality, our universe. I will give a text of wikipedia to show the difference: Up til the 15t yeirhunder Scottis (modren furm Scots) wis the name o "Gaelic", the Celtic leid o the aunshint Scots. English: Up till the 15th sanctuary Scottis (modern form Scots) was the name of "Gaelic", the Celtic language of the ancient Scots. As we see, there isn't much difference. Except for the words yeirhunder and leid, someone who speaks English would be able to perfectly understand this. Our fictional 'Sclotlèn' though, developed different. Because they wanted to seperate themselves a lot from English-speakers, they invented a new writing system with the same Latin letters: a = o r = s n = v m = w b = t k = m w = l u = a p = c t = w i = u h = x s = z o = i e = i Up til the 15t yeirhunder Scottis (modren furm Scots) wis the name o "Gaelic", the Celtic leid o the aunshint Scots. Ac wul wxi 15w yeusxavdes Zcowwuz (midsiv fusw Zciwz) luz wxi vowi i "Goiluc", wxi Cilwuc liud i wxi oavzxuvw Zciwz. As you can notice, this text has become completely unreadable for English-speakers, by replacing all the letters. This is where we have reached my question. Has this ever happened in language history? That a language which is actually quite similar, has become unreadable with replacement or addition of letters in the one alphabet? I am not talking about Turkish here which has adopted the Latin alphabet and which became unreadable, because here we have a completely different alphabet. I am talking about the same alphabet, in which it has become unreadable. My second question is: Has it ever happened that a language completely changed because this change of letters is pronounced too. So the text here would be pronounced like: Ac wul wkhi 15v yuskhavdes Zcovuz (midsiv fusv Zcivz) luz wkhi vovi i "Goiluc", vkhi Cilvuc liud i vkhi oavzkhuvv zcivz.