The traditional or classical Mongolian alphabet, sometimes called Hudum 'traditional' in Oirat in contrast to the Clear script (Todo 'exact'), is the original form of the Mongolian script used to write the Mongolian language. It does not distinguish several vowels (o/u, ö/ü, final a/e) and consonants (t/d, k/g, sometimes ž/y) that were not required for Uyghur, which was the source of the Mongol (or Uyghur-Mongol) script. The result is somewhat comparable to the situation of English, which must represent ten or more vowels with only five letters and uses the digraph th for two distinct sounds. Ambiguity is sometimes prevented by context, as the requirements of vowel harmony and syllable sequence usually indicate the correct sound. Moreover, as there are few words with an exactly identical spelling, actual ambiguities are rare for a reader who knows the orthography.