|There are five English front vowels.
Since both  and  are high front vowels, we need to identify the articulatory difference that distinguishes them. As it turns out,  is a bit higher and more forward than , a contrast that is referred to as the tense/lax distinction.  is tense in that the root of the tongue is more advanced than with , which is lax; correspondingly,  is tense and  is lax.
The English vowel system also has sequence of back vowels that differ from each other in terms of height and tenseness. In addition, some of the back vowels of English involve rounding the lips--a feature that no English front vowel manifests.