I searched, the WWW (which is probably what I should have done first to begin with -- sorry) and found the following:
One website (
http://www.antimoon.com/forum/t7020.htm) says:
I don't know how 'clerk' and 'derby' were pronounced in Middle English, but according to 19th century British grammarian, Alfred Ayres, the traditional pronunciation of 'e' before 'r', followed by another consonant, is /a:/ as in 'dark'. Thus, words like 'merchant', 'service', and 'servant' were pronounced as if written as 'marchant' (compare with Anglo-French 'marchaunt'), 'sarvice', and 'sarvant'. Modern RP English pronunciations of 'clerk', 'derby', 'Berkeley', and 'sergeant' (also in AmE) still retain this pronuncation rule.
Stephen Booth in his book on Shakespeare's sonnets states that Renaissance writers and printers used 'ar' and 'er' interchangeably, and early editions of the Oxford English Dictionary had words like 'partain', 'pert', 'pertake', and 'pertener' listed as variants of 'pertain', 'part', 'partake', and 'partner'.
I haven't any sense of from what era "traditional pronunciations" come, but if they come from roughly the 1800s, the actual "right answer" is likely some mix of the statements above and the ones below.
Another website (
http://english.stackexchange.com/que...d-derby-evolve) said the following:
It's the result of the same process (that is, erroneous pronunciation) whereby "learn" becomes "larn" in some (very) nonstandard American dialects. One feature of uneducated speech in England around the 1800's was a tendency to pronounce the "er" sound of words like "clerk" as the "ar" sound of "clark". The phenomenon was sufficiently widespread that the English novelist Henry Fielding used pronunciations like "sarvis" for "service", "sartain" for "certain", and "parson" for "person" in the speech of characters meant to seem vulgar or unintelligent. Due to the overwhelming influence of such people in England (that is, the uneducated), these previously unacceptable pronunciations eventually became standard for some words, like Derby, Berkeley, and clerk...
Source(s): J.C. Wells, Accents of English