Nonetheless, the concept that vowel qualities are determined primarily by tongue position and lip rounding continues to be used in pedagogy, as it provides an intuitive explanation of how vowels are distinguished.
The International Phonetic Alphabet defines seven degrees of vowel height, but no language is known to distinguish all of them without distinguishing another attribute:
The International Phonetic Alphabet defines five degrees of vowel backness:
Although some English dialects have vowels at five degrees of backness, there is no known language that distinguishes five degrees of backness without additional differences in height or rounding.
Roundedness is named after the rounding of the lips in some vowels. Because lip rounding is easily visible, vowels may be commonly identified as rounded based on the articulation of the lips. Acoustically, rounded vowels are identified chiefly by a decrease in F2, although F1 is also slightly decreased.
Nonetheless, even in those languages there is usually some phonetic correlation between rounding and backness: front rounded vowels tend to be more front-central than front, and back unrounded vowels tend to be more back-central than back. Thus, the placement of unrounded vowels to the left of rounded vowels on the IPA vowel chart is reflective of their position in formant space.
Rhotic vowels are the "R-colored vowels" of American English and a few other languages.
Rounding is generally realized by a decrease of F2 that tends to reinforce vowel backness. One effect of this is that back vowels are most commonly rounded while front vowels are most commonly unrounded; another is that rounded vowels tend to plot to the right of unrounded vowels in vowel charts. That is, there is a reason for plotting vowel pairs the way they are.dad, dada, dado, dead, deed, did, died, diode, dodo, dud, dude, odd, add
The differences in pronunciation of vowel letters between English and its related languages can be accounted for by the Great Vowel Shift. After printing was introduced to England, and therefore after spelling was more or less standardized, a series of dramatic changes in the pronunciation of the vowel phonemes did occur, and continued into recent centuries, but were not reflected in the spelling system. This has led to numerous inconsistencies in the spelling of English vowel sounds and the pronunciation of English vowel letters (and to the mispronunciation of foreign words and names by speakers of English).