(The use of the term tone in this sense should not be confused with the tone of tone languages, such as Chinese.) These terms ("the Three Ts") have been used in more recent work, though they have been criticized for being difficult to remember. American systems such as ToBI also identify contrasts involving boundaries between intonation phrases (Halliday's tonality), placement of pitch accent (tonicity), and choice of tone or tones associated with the pitch accent (tone). Example of phonological contrast involving placement of intonation unit boundaries (boundary marked by comma): Example of phonological contrast involving placement of tonic syllable (marked by capital letters): Example of phonological contrast (British English) involving choice of tone (\ = falling tone, \/ = fall-rise tone) There is typically a contrast involving tone between wh-questions and yes/no questions, the former having a falling tone (e.g. "Where did you \PUT it?") and the latter a rising tone (e.g. "Are you going /OUT?"), though studies of spontaneous speech have shown frequent exceptions to this rule. Tag questions asking for information are said to carry rising tones (e.g. "They are coming on Tuesday, /AREN'T they?") while those asking for confirmation have falling tone (e.g. "Your name's John, \ISN'T it."). History of English pronunciation The pronunciation system of English has undergone many changes throughout the history of the language, from the phonological system of Old English, to that of Middle English, through to that of the present day. Variation between dialects has always been significant. Former pronunciations of many words are reflected in their spellings, as English orthography has generally not kept pace with phonological changes since the Middle English period. The English consonant system has been relatively stable over time, although a number of significant changes have occurred. Examples include the loss (in most dialects) of the and sounds still reflected by the in words like night and taught, and the splitting of voiced and voiceless allophones of fricatives into separate phonemes (such as the two different phonemes represented by ). There have also been many changes in consonant clusters, mostly reductions, for instance those that produced the usual modern pronunciations of such letter combinations as , and . The development of vowels has been much more complex. One of the most notable series of changes is that known as the Great Vowel Shift, which began around the late 14th century. Here the and in words like price