【正确答案】正确答案：As the vowels can not be described in the same way as the consonants, a system of cardinal vowels has been suggested to get out of this problem. The cardinal vowels are a set of vowel qualities arbitrarily defined, fixed and unchanging, intended to provide a frame of reference for the description of the actual vowels of existing languages. The cardinal vowels are abstract concept. The cardinal vowel diagram is a set of hypothetical positions for vowels used as reference points. The description of English vowels needs to fulfill four basic requirements: (1) the height of tongue raising (high, middle or low) ; (2) the position of the highest part of the tongue (front, central , back) ; (3) the length or tenseness of the vowel(tense vs. lax or long vs. short) ; (4) lip-rounding (rounded vs. unrounded). For example, [ i: ] : high, front, tense, unrounded vowel. [ u ] : high, back, lax, rounded vowel.