It is very rare that one can date a certain change precisely. What is much more common is an approximate dating with a century or more. But far more positive evidence can be gained for relative rather than absolute chronology by which is meant that two or more changes can be put in chronological order relative to each other. This is usually possible because the result of one change would have been different if it had preceded or followed the other. Here are some examples to illustrate what is meant.
Palatalisation and i-umlaut in Old English
|1)||palatalisation cinn||>||chin [tʃɪn]|
|2)||i-umlaut kuning||>||kyning [kynɪŋ]|
It is obvious that palatalisation preceded i-umlaut otherwise the pronunciation of the word for ‘king’ would be [tʃɪŋ], that is the process of palatalisation would have appeared to have become inactive before i-umlaut set in so that those words which experienced i-umlaut did not go through palatalisation.
VOWEL SHORTENINGS IN ENGLISH A tendency which can be observed in the history of English is for long /u:/ to be shortened. This starts in the early modern period and continues to the present-day. The forms affected by this change differ in their realisations today depending on when the shortening took effect with them.
|a)||food /o:/||>||/u:/ (no shortening)|
|b)||blood /o:/||>||/u:/||>||/u/||>||/ʌ/ (early shortening with lowering)|
|c)||took /o:/||>||/u:/||>||>||/u/ (late shortening)|
|d)||room /o:/||>||/u:/||>||>||/u/ (present shortening)|
With these changes one can specify the phonotactic environment in which they took place. The earliest shortening affects /u:/ before /d/ and took place before the general lowering of /u/ to /ʌ/ in southern English in the early modern period and hence underwent this change. After this shortening came that of /u:/ before /k/. This took place after the lowering of /u/ to /ʌ/ had become inactive, hence the pronunciation /bʊk/ for book and not /bʌk/.
Finally the shortening before /m/ occurs. This shortening has not been completed yet as can be seen from words which have variable realisations in Received Pronunciation: broom /brʊm/ or /bru:m/. The earlier shortenings may or may not be present in different varieties of English for instance northern English and Scottish English do not have the lowering of /u/ to /ʌ/; Irish English does not have the shortening before velars in all cases, cf. cook /ku:k/ and not /kʊk/.
GREAT VOWEL SHIFT AND FRENCH LOANS The long vowel shift in English is a process which began in the late Middle English period. By this time most of the French loans (Norman and Central French) had already entered the language and thus underwent the Great Vowel Shift, e.g. doubt /daut/ from an earlier /du:t/. However, a significant number of loans did not undergo the shift and so one must assume that they were borrowed after the Great Vowel Shift had been completed.
|a)||divine /i:/||/ai/||machine /i:/ (no shift)|
|b)||gout /u:/||/au/||rouge /u:/ (no shift)|
One must also consider the operation of later analogy. There are a few instances where orthographic ou is realised as /au/ for example with route /raut/ in American English (sometimes) whereas British English still has /ru:t/.