How did the pronunciation of the word "derby" evolve?

Brits say "dar-bee" for both the town and the race, but Americans pronounce it as it's spelled.

Did Brits used to say "der-bee" too and that's why it's spelled that way but they changed over time?

Was it always pronounced like in the UK but American pronunciation followed the spelling for some odd reason?

Is it borrowed from some other language so there's a disconnect between the spelling and the UK pronunciation?

Solution 1:

This website says

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

So a more general sound change that took "er" to "ar" was present in some lower-class British accents when Henry Fielding wrote Tom Jones (ca. 1750). This 1849 dictionary found in Google books indicates that this pronunciation had for the most part vanished a hundred years later¹. The pronunciation now persists in only a handful of words:

Berkeley, clerk, derby, Hertfordshire, sergeant, varsity.

I take the liberty of including varsity here because it was originally short for university. However, university has reverted to the standard pronunciation, while varsity has switched to the phonetic spelling.

¹ Interestingly, the dictionary also says that many Americans pronounced sergeant with "er".

Solution 2:

John Walker (1732 - 1807) has the following remark in his Critical Pronouncing Dictionary (London, 1791):

• There is a remarkable exception to the common sound of this letter [e] in the words clerk, serjeant, and a few others, where we find the e pronounced like the a in dark and margin. But this exception was, I imagine, till within these few years, the general rule of sounding this letter before r, followed by another consonant. Thirty years ago everyone pronounced the first syllable of merchant like the monosyllable march, and as it was anciently written marchant.
  Service and servant are still heard among the lower order of speakers, as if written sarvice and sarvant: and even among the better sort, we sometimes hear, Sir, your sarvant; though this pronunciation of the word singly would be looked upon as a mark of the lowest vulgarity.
  The proper names Derby and Berkeley still retain the old sound as if written Darby and Barkeley: but even these, in polite usage, are getting into the common sound, nearly as if written Durby and Burkeley. As this modern pronunciation of the e has a tendency to simplify the language by lessening the number of exceptions, it ought certainly to be indulged.

When it comes to some words spelt with er, it seems that the change from /ɑ:/ to /ɜ:/ (I use Wells's notation in the 3rd edition of the LPD) in British English happened in the second half of the eighteenth century and was completed by the 1790s, with a few exceptions (clerk, sergeant...) that remain to this day.

J. C. Wells in Accents of English (1982) says the following:

• There are quite a few instances where preconsonantal or final Middle English /ɛr/ has given a current standard pronunciation with the vowel of START rather than that of NURSE, for example far, star, heart. There seems to have been a period of some fluctuation in such words, with the tendency towards spelling pronunciation perhaps playing a rôle; one consequence is the handful of present-day well-known British/American differences — clerk, Derby, Berkeley with RP /ɑ:/ but GenAm /ɝ:/. Other variants with START for NURSE or vice versa are still found in conservative regional speech both in England and elsewhere, including Appalachia. In England, both Derby and clerk are quite often pronounced with /ɜ:/ by working-class speakers. In the eighteenth century the novelist Fielding put the pronunication 'sarvis, sartain, parson' into the speech or writing of characters we are intended to consider vulgar. The last-mentioned form, though obsolescent or obsolete now in the sense 'person', has nevertheless lived on in the doublet word parson 'clergyman'; the two words are etymologically identical. We still have varmin(t) as a rustic variant of vermin. The form varsity is still alive in some places as an abbreviation of university. And I'll larn you remains as a jocular pseudodialectalism meaning 'I'll teach you a lesson you won't forget', a witness to the once widespread usage whereby learn had the START vowel — a usage still frequently encountered, for example, in popular Jamaican English.

A. C. Gimson, pronounced /ˈɡɪmsən/, in an Introduction to the Pronunciation of English (third edition, 1980), which is a description of RP, sheds further light on the issue and on the NURSE merger:

• The great majority of cases of present English /ɜ:/ [American English /ɝ:/] derive from Middle English [ɛ] + /r/ (virtue = vertue, earth, heard, fern) or ME [ɪ] + /r/ (shirt, birth, myrrh); ME [ʊ] + /r/ (word, journey, spur). ME [ɛ], [ɪ], and [ʊ] (> [ʌ] with loss of lip-rounding) all centralized before final /r/ or /r/ + consonant, so that the pronunciation [ər] for all three was incipient in the London region in the sixteenth century, though contemporary grammarians still often insisted upon the earlier vowel quality especially in the case of ME [ɛ] and [ʊ] words. With the loss of /r/ in post-vocalic positions in the eighteenth century, the present English long /ɜ:/ was reached.

He cites as one of the sources for RP /ɑ:/:

• through loss of post-vocalic /r/ in the eighteenth century, short [a] ot [æ] > /ɑ:/ (charm, march); the [a] or [æ] in question may often result from ME [ɛ] (far, star, heart) as well as an earlier French [ɛ] (farm, clerk, sergeant).

To sum up, Middle English [ɛ] split into the START and the NURSE vowels with some of the START vowel words, like servant, person, learn etc. changing to the NURSE set in the second half of the eighteenth century as remarked upon by Walker. Many of them, like farm, heart, star, did not undergo any change and kept the START vowel. Strikingly, some spelt with er such as clerk, Derby, Berkeley remained unchanged in British English with /ɑ:/ but joined the NURSE set in American English. Sergeant or serjeant stands alone as being spelt with er and having the START vowel in American English too.