Tongue position maps to vowel sound
The IPA Vowel Space is a trapezoid representing the human vocal tract. The horizontal axis shows tongue backness, from front (like 'ee' in 'fleece') to back (like 'oo' in 'goose'). The vertical axis shows tongue height, from high (close to the roof of the mouth) to low (near the floor of the mouth). The trapezoid shape reflects the fact that the vocal tract itself is wider at the back and narrower at the front. Each vowel's position corresponds to approximately where the tongue sits when that vowel is produced.
English has roughly 14-20 vowel sounds, distributed across the vowel space. Other languages use the space differently. Some languages use fewer vowels and leave large areas of the space unexploited. Others divide the space more finely, adding lip rounding as an additional dimension to create even more vowel contrasts.
Vowel systems and acoustic targets
Vowels can be described in three dimensions: tongue height, tongue backness, and lip rounding. Some vowels are rounded (lips are protruded) and others are unrounded. This three-dimensional space means languages can create dozens of vowel distinctions. The extremes of the vowel space, the cardinal vowels, provide reference points phoneticians use to describe any vowel in the world. A vowel between cardinal 1 (like English 'fleece') and cardinal 2 (like French 'patte') can be precisely described by its position relative to these reference points.