It is the standard British accent that has drastically changed in the past two centuries, while the typical American accent has changed only subtly。
在过去的两个世纪，标准的英国口音已发生巨大变化，而典型的美国口音则只有微妙改变。

Traditional English, whether spoken in the British Isles or the American colonies, was largely "rhotic." Rhotic speakers pronounce the "R" sound in such words as "hard" and "winter," while non-rhotic speakers do not. Today, however, non-rhotic speech is common throughout most of Britain. For example, most modern Brits would tell you it's been a "hahd wintuh."
传统英语，无论是在英伦三岛或美国殖民地，其发音大部分都带有“卷舌音” 。带卷舌音的人在说 "hard(艰难)" 和 "winter(冬季)" 这类词时，总是发出“R”音，而不带卷舌音的人则不然。然而今天，在英国大部分地区，人们说话通常都不带卷舌音。例如，大部分现在英国人会跟你说，这是一个“hahd wintuh(严冬)”。

It was around the time of the American Revolution that non-rhotic speech came into use among the upper class in southern England, in and around London. According to John Algeo in "The Cambridge History of the English Language" (Cambridge University Press, 2001)， this shift occurred because people of low birth rank who had become wealthy during the Industrial Revolution were seeking ways to distinguishthemselves from other commoners; they cultivated the prestigious non-rhotic pronunciation in order to demonstrate their new upper-class status。
大约在美国独立战争时期，在伦敦及附近地区，英格兰南部的上层阶级之间开始用非卷舌音讲话。据约翰·艾尔吉奥在《剑桥英语史》(剑桥大学出版社，2001年)的说法，之所以发生这种转变，因为出生于低阶层的人们在工业革命时期已发家致富，他们极力寻求使自己有别于其他平民的方法；他们逐步养成久负盛名的非卷舌音发音，以显示他们的新贵地位。

"London pronunciation became the prerogative of a new breed of specialists — orthoepists and teachers of elocution. The orthoepists decided upon correct pronunciations, compiled pronouncing dictionaries and, in private and expensive tutoring sessions, drilled enterprising citizens in fashionable articulation," Algeo wrote。
艾尔吉奥写道：“伦敦发音成为专家们(正音学研究者和朗诵教师)对一种新口音的特权。正音学研究者们决定修正发音，编纂发音词典，并在私下收费高昂的辅导课上，用流行的发音方式对一些努力进取的公民进行训练”。

The lofty manner of speech developed by these specialists gradually became standardized — it is officially called "Received Pronunciation" — and it spread across Britain. However, people in the north of England, Scotland and Ireland have largely maintained their traditional rhotic accents。
由这些专家们开发的高傲讲话方式逐渐成为标准化 -被正式称为“标准发音” -在英国各地传播开来。不过，在英格兰北部、苏格兰和爱尔兰的人们大部分都还保持其传统带卷舌音的口音。

Most American accents have also remained rhotic, with some exceptions: New York and Boston accents have become non-rhotic. According to Algeo, after the Revolutionary War, these cities were "under the strongest influence by the British elite."
大多数美国人的口音也保持卷舌音，当然也有例外：纽约和波士顿口音已成为非卷舌音。据艾尔吉奥的说法，独立战争后，这些城市“受英国精英们的影响最大。”