Diphthong
A monophthong is a simple vowel sound that a person does not have to move his or her mouth to make, like the "oo" sound in "book." In a diphthong, the person combines two different monophthongs, as with the "oi" sound in the word "oil". The speaker starts with the mouth in the position to make an "o" sound, then quickly moves the mouth to make a hard "e" sound. Another example is the "ou" sound in the word "house". The mouth starts out making a sound like the soft "a" sound in "flat", then moves to make the a hard "oo" sound like the one in "caboose".