Tenseness/laxness is one of the three distinctive features of vowels. This blog post will explain the two contractions, used in English, and list the English vowels that happen with each.
The tenseness or laxness of a vowel is the distinction of whether the tongue is tense (periphery) or lax (centralized).
Tense = tense vowels occur when there is a larger degree of tension in the mouth, these vowels are usually longer. Ex. fr(ee), sh(oe), g(o), d(a)y,
Lax = lax vowels occur when there is a small degree of tension for the tongue. Ex. p(i)t, f(oo)t, p(e)t, (a)bove, n(u)t, p(aw), b(a)d, b(a)lm