Explain the difference between rhotic and non-rhotic dialects of English.

Rhoticity in English is the pronunciation of the historical rhotic consonant /r/ by English speakers. The presence or absence of rhoticity is one of the most prominent distinctions by which varieties of English can be classified. In rhotic varieties, the historical English /r/ sound is preserved in all pronunciation contexts. In non-rhotic varieties, speakers no longer pronounce /r/ in postvocalic environments—that is, when it is immediately after a vowel and not followed by another vowel. For example, in isolation, a rhotic English speaker pronounces the words hard and butter as /ˈhɑːrd/ and /ˈbʌtər/, whereas a non-rhotic speaker "drops" or "deletes" the /r/ sound, pronouncing them as /ˈhɑːd/ and /ˈbʌtə/.[a] When an r is at the end of a word but the next word begins with a vowel, as in the phrase "better apples", most non-rhotic speakers will pronounce the /r/ in that position (the linking R), since it is followed by a vowel in this case.

The rhotic varieties of English include the dialects of South West England, Scotland, Ireland, and most of the United States and Canada. The non-rhotic varieties include most of the dialects of modern England, Wales, Australia, New Zealand, and South Africa. In some varieties, such as those of some parts of the southern and northeastern United States, rhoticity is a sociolinguistic variable: postvocalic r is deleted depending on an array of social factors, such as being more correlated today with lower socioeconomic status, greater age, certain ethnic identities, and less formal speaking contexts.

Evidence from written documents suggests that loss of postvocalic /r/ began sporadically during the mid-15th century, although these /r/-less spellings were uncommon and were restricted to private documents, especially ones written by women in England. In the mid-18th century, postvocalic /r/ was still pronounced in most environments, but by the 1740s to 1770s it was often deleted entirely, especially after low vowels. By the early 19th century, the southern British standard was fully transformed into a non-rhotic variety, though some variation persisted as late as the 1870s.

In the 18th century (and possibly 17th century), the loss of postvocalic /r/ in British English influenced southern and eastern American port cities with close connections to Britain, causing their upper-class pronunciation to become non-rhotic while the rest of the United States remained rhotic. Non-rhotic pronunciation continued to influence American prestige speech until the 1860s, when the American Civil War began to shift America's centers of wealth and political power to rhotic areas with fewer cultural connections to the old colonial and British elites. In the United States, non-rhotic speech continued to hold some level of prestige up until the mid-20th century, but rhotic speech in particular became prestigious nationwide rapidly after the Second World War, reflected in the national standard of radio and television since the mid-20th century embracing historical /r/.

In rhotic English dialects, the historical English /r/ sound is preserved in all pronunciation contexts. In non-rhotic English dialects, speakers no longer pronounce /r/ in postvocalic environments—that is, when it is immediately after a vowel and not followed by another vowel.  For example, in isolation, a rhotic English speaker pronounces the words hard and butter as /ˈhɑːrd/ and /ˈbʌtər/, whereas a non-rhotic speaker "drops" or "deletes" the /r/ sound, pronouncing them as /ˈhɑːd/ and /ˈbʌtə/.  The rhotic varieties of English include the dialects of South West England, Scotland, Ireland, and most of the United States and Canada. The non-rhotic varieties include most of the dialects of modern England, Wales, Australia, New Zealand, and South Africa.