Vowels beside dots are: unrounded • rounded
The front rounded vowels defined by the IPA include:
Lip rounding is produced by bringing the corners of the lips together and protruding them forward. This is harder to do while producing low or open vowels, since the lips are being stressed vertically. This may explain why low vowels are usually unrounded. Roundedness is usually associated with back vowels. This helps to reinforce the low-pitched acoustic overtones associated with back vowels. This makes rounded back vowels more distinct from other vowels.
The high vowel [y] is the most common, while the low vowel [ɶ] is extremely rare. This is consistent with the general correlation between rounding and vowel height.
Front rounded vowels usually occur in languages with vowel systems that distinguish a higher than average number of different vowel qualities. Typically, when a front rounded vowel occurs, the inventory of vowels includes an unrounded front vowel and a rounded back vowel of similar height. For example, many languages with [y], such as French, Albanian, or Dagur, also have [i] and [u]. Because of this, and what's known about many languages with front rounded vowels, front rounded vowels typically reflect a development from a vowel system that previously had fewer members.
Language families in which front-rounded vowels are common are:
Many of the Uralic, Mongolic, and Turkic languages which contain front rounded vowels also have vowel harmony systems, such as rounding or backness harmony. The processes which bring about fronting or roundedness harmony may be important in introducing front rounded vowels into a language's inventory.
Front rounded vowels can also develop independently of vowel harmony. In French, [y] is the result of an older [u] moving forward while remaining rounded. French re-developed [u] by moving [o] upwards. For example, the familiar second-person pronoun tu, pronounced [ty], is spelled the same as its Latin source tucode: lat promoted to code: la which had a back vowel, and the plural or polite second-person pronoun vous is pronounced with [u], but it derives from Latin voscode: lat promoted to code: la which had a mid vowel. Some varieties of modern English are similarly fronting [u]. In cases like these the creation of front rounded vowels is independent of adjacent sounds in the word.
In other languages front rounded vowels evolved as previously back vowels became fronted by adjacent segments. For example, in German, back vowels became fronted when followed by high front vowels. The front vowels causing this change were often in inflectional or derivational endings and were then lost or changed into mid vowels. In the different Chinese languages, [y] is often derived from [ut]. The tongue must be moved forward to pronounce the [t], so the [u] can shift forward as well. If the [t] is lost then the change from [ut] to [y] is complete.
Considering the different historical scenarios which can give rise to front rounded vowels, it is notable that they are relatively concentrated in a particular geographical region. It's likely that the presence of front rounded vowels in some languages can facilitate phonetically-motivated processes in other nearby languages.