THE RELATION IS EXAMINED BETWEEN SIMPLE VOWEL SOUNDS IN ENGLISH AND VOWELS ASSOCIATED WITH GLIDES, OR SEMIVOWELS, SOMETIMES REFERRED TO AS "COMPOUND PHONEMES." THESE COMPLEX VOWEL NUCLEI PARTICIPATE IN MORPHOPHONEMIC ALTERNATIONS WITH SIMPLE VOWEL NUCLEI, AS FOR EXAMPLE, IN THE ALTERNATION OF VOWEL NUCLEI IN THE PAIR "SLEEP/SLEPT." THE SYSTEM UNDERLYING SUCH REGULAR ALTERNATIONS IS DESCRIBED AND RULES ARE GIVEN TO ACCOUNT FOR THE PHONETIC SHAPES OF THE ALTERNATIONS. THE DEVELOPMENT OF SUCH A SYSTEM RESTS ON THE ASSUMPTIONS THAT (1) FORMS PHONETICALLY DIFFERENT BUT MORPHOPHONEMICALLY RELATED ARE BASED ON THE SAME UNDERLYING FORM, AND (2) THE UNDERLYING SYSTEM COULD BE EVALUATED BY CONSIDERATIONS OF OPTIMAL SIMPLICITY IN THE SYSTEM AS WELL AS IN THE RULES FOR DERIVATION OF SUPERFICIAL PHONETIC SHAPES FROM UNDERLYING MORPHOPHONEMIC SHAPES. ENGLISH COMPLEX AND SIMPLE VOWEL NUCLEI ARE CLASSIFIED IN TERMS OF DISTINCTIVE FEATURES AND THE TWO SETS ARE DISTINGUISHED BY THE FEATURE OF TENSENESS VERSUS LAXNESS IN THE VOWELS UNDERLYING THE NUCLEI. THESE RULES ACCOUNT SYNCHRONICALLY FOR PHENOMENA WHICH ARE THE RESULTS OF THE SO-CALLED "GREAT VOWEL SHIFT" AND CAN BE ACCOUNTED FOR SYNCHRONICALLY BY ONE RULE FOR THE TENSE VOWELS AND ANOTHER FOR THE LAX VOWELS. (KL)