Cardinal vowel

Vowel sound produced when the tongue is in an extreme position, either front or back, high or low. See Daniel Jones. Cardinal vowels are obtained by dividing the "operture space" between the closest vowels [i] and [u] and the most open vowel [a] in four equal "degrees" of operture: close (high tongue position), close-mid, open-mid, and open (low tongue position). These degrees of operture plus the front-back distinction originates 8 reference articulatory points, and the 16 IPA cardinal vowels when the position of lips are considered (rounded/unrounded vowels).

cardinal	SAMPA	description
1	[i]	front close unrounded
2	[e]	front close-mid unrounded
3	[E]	front open-mid unrounded
4	[a]	front open unrounded
5	[A]	back open unrounded
6	[O]	back open-mid rounded
7	[o]	back close-mid rounded
8	[u]	back close rounded
9	[y]	front close rounded
10	[2]	front close-mid rounded
11	[9]	front open-mid rounded
12	[&]	front open rounded
13	[Q]	back open rounded
14	[V]	back open-mid unrounded
15	[7]	back close-mid unrounded
16	[M]	back close unrounded

Note that although [a] is listed as a front vowel, it is considered as a central vowel by many.