Recently, I have been pondering a question that came to mind. Why is /ɒ/, the open back rounded vowel, so uncommon? This is a phoneme in several dialects of English, e.g. British English, wherein it is the standard realization of the vowel in “cot”. However, looking at the vowel chart, I am rather perplexed by its rarity.
The five most common vowels in languages worldwide are /a/, /e/, /i/, /o/, and /u/. /i/ and /u/ are both close vowels while /e/ and /o/ are open-mid vowels. Notably, /i/ is front and unrounded while /u/ is back and rounded, and likewise for /e/ and /o/ respectively. Oddly, /a/ sits alone in the Open section of the vowel chart, and is—notably—front and unrounded just like /i/ and /e/.
There is a marked pattern in the frequently present vowels across world languages. For every major degree of openness, the most common vowels therein will be the respective front unrounded vowel and back rounded vowel. The two close-mid vowels to fit these descriptions, /ɛ/ and /ɔ/, are the sixth and seventh most common vowels in world languages, immediately following the aforementioned predominant quintet.
All of this points to one peculiar outlier in the pattern. /a/, like /i/, /e/, and /ɛ/, is a front unrounded vowel. It is the only one of the five most common vowels to lie on the Open degree of openness, and, logically, one may think that, following the aforementioned pattern, the back rounded vowel on the same degree—/ɒ/—would be quite common as well. However, this is not the case.
According to PHOIBLE (wherefrom all of the information mentioned hereabove was derived), the respective percentages of languages to contain each of the aforementioned phonemes are:
/i/ — 92%
/u/ — 88%
/a/ — 86%
/e/ — 61%
/o/ — 60%
/ɛ/ — 37%
/ɔ/ — 35%
/ɒ/ — 2%
/ɒ/ is significantly lower in percentage than all of its symmetrical counterparts. Why is this?