In Black Africa, Australia, the Caribbean (except for Barbados), England (apart from the south-west), New Zealand, South Africa, the southern U.S. states, the Boston area of New England, New York City vernacular speech, Wales, and the Black English vernacular in the U.S., the r is non-rhotic.
Not only do sounds change geographically and socially, they also change with the passing of time, and with dramatic events such as a long-time occupation by another linguistic group (The Philippines are a good example, as they were occupied by Spanish-speaking and then English-speaking invaders not so long ago).
Pedagogically, this flux and this variety of influences render the learning of English pronunciation not so easy an enterprise for learners of English as a foreign language (Which teacher's pronunciation is the model to imitate?). The problem is compounded by the natural tendency of learners to transfer pronunciation and intonation from their native language (such as the rhotocization of Arabic and the Romance languages, and the non-rhotocization of Black African, Chinese, and Japanese dialects--not to omit BBC English).
Conventionally, the history of the English language is divided into three broad periods, viz. Old English (c. 450-1100), Middle English (c. 100-1500), and Modern English (from c. 1500). Some linguists may also use the term World English (1960s) to denote English as a world language suffering (or enjoying) changes differen