own. As for transcriptions meant to approximate the pronunciation, Tibetan pinyin is the official romanization system employed by the government of the People's Republic of China, while English language materials use the THL transcription system. Certain names may also retain irregular transcriptions, such as Chomolungma for Mount Everest. Phonology The following summarizes the sound system of the dialect of Tibetan spoken in Lhasa, the most influential variety of the spoken language. Vowels Tournadre and Sangda Dorje describe eight vowels in the standard language: Three additional vowels are sometimes described as significantly distinct: or , which is normally an allophone of ; , which is normally an allophone of ; and (an unrounded, centralised, mid front vowel), which is normally an allophone of . These sounds normally occur in closed syllables; because Tibetan does not allow geminated consonants, there are cases in which one syllable ends with the same sound as the one following it. The result is that the first is pronounced as an open syllable but retains the vowel typical of a closed syllable. For instance, zhabs (foot) is pronounced and pad (borrowing from Sanskrit padma, lotus) is pronounced , but the compound word, zhabs pad is pronounced . This process can result in minimal pairs involving sounds that are otherwise allophones. Sources vary on whether the phone (resulting from in a closed syllable) and the phone (resulting from through the i-mutation) are distinct or basically identical. Phonemic vowel length exists in Lhasa Tibetan but in a restricted set of circumstances. Assimilation of Classical Tibetan's suffixes, normally ‘i (འི་), at the end of a word produces a long vowel in Lhasa Tibetan; the feature is sometimes omitted in phonetic transcriptions. In normal spoken pronunciation, a lengthening of the vowel is also frequently substituted for the sounds and when they occur at the end of a syllable. The vowels , , , , and each have nasalized forms: , , , , and , respectively, which historically results from , , etc. In some unusual cases, the vowels , , and may also be nasalised. Tones The Lhasa dialect is usually described as having two tones: high and low. However, in monosyllabic words, each tone can occur with two distinct contours. The high tone can be pronounced with either a flat or a falling contour, and the low tone can be pronounced with either a flat or rising-falling contour, the