Backness

Although English has vowels at five degrees of backness, there is no known language that distinguishes five degrees of backness without additional differences in height or rounding.

Daniel Jones developed the cardinal vowel system to describe vowels in terms of the features of tongue height (vertical dimension), tongue backness (horizontal dimension) and roundedness (lip articulation).

For instance, Altaic languages are proposed to have a rounding harmony superimposed over a backness harmony.

From vowel harmony it follows that the initial syllable of each single (non-compound) word controls the frontness or backness of the entire word.

In this type of vowel harmony, the position of the root of the tongue in regards to backness is the phonetic basis for the distinction between two harmonizing sets of vowels.

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.

No other parameter, even backness or rounding (see below), is used in all languages.

The position of the highest point is used to determine vowel height and backness.