The close back unrounded vowel, or high back unrounded vowel, is a type of vowel sound used in some spoken languages. Acoustically it is a close back-central unrounded vowel. The symbol in the International Phonetic Alphabet that represents this sound is ⟨ɯ⟩. Typographically a turned letter m, given its relation to the sound represented by the letter u it can be considered a u with an extra "bowl". The sound is sometimes referred to as "unrounded u".
The IPA prefers terms "close" and "open" for vowels, hence the name of this article. However, a large number of linguists, perhaps a majority, prefer the terms "high" and "low".
Its vowel height is close, also known as high, which means the tongue is positioned as close as possible to the roof of the mouth without creating a constriction that would be classified as a consonant.
Its vowel backness is back, which means the tongue is positioned as far back as possible in the mouth without creating a constriction that would be classified as a consonant. Note that unrounded back vowels tend to be centralized, which means that often they are in fact near-back.
It is unrounded, which means that the lips are not rounded.
Asu, Eva Liina; Teras, Pire (2009), "Estonian", Journal of the International Phonetic Association39 (3): 367–372, doi:10.1017/s002510030999017x
Bowerman, Sean (2004), "White South African English: phonology", in Schneider, Edgar W.; Burridge, Kate; Kortmann, Bernd; Mesthrie, Rajend; Upton, Clive, A handbook of varieties of English, 1: Phonology, Mouton de Gruyter, pp. 931–942, ISBN3-11-017532-0