No, dipthongs aren't a special kind of undies, they're vowels. As English happens to be one of the most befuddling languages, 'in my line of work' I learn something new about the English sound/spelling system everyday. Yesterday a second grade teacher set me straight. We were discussing dipthongs, those tantalizing 'gliding' vowel sounds like 'oi' and 'ow.' ((Picture Eliza Dolittle right now. She belted out many a dipthong.))
For good reason, vowels sounds are the most difficult for children to learn to read and spell. Even though there are only 44 sounds in English, one sound can be represented by many different spellings. When you produce a dipthong, your 'articulators' transition to make two vowel sounds welded into one, represented by two letters (oo, ou, oi, ow). Vowel sounds are 'open' sounds, whereas consonants are produced by blocking air.
I opined that all the dipthongs were long vowels, because of the two letter representation, plus the two sounds welding. The second grade teacher said: "I thought all long vowel sounds have to say their name?" Then the reading teacher pointed out the 'oo' in book is a short sound. Hmm. Turns out we were all right, in varied ways. That's English for you. Dipthongs can be both short and long vowel sounds (for example: book vs school and growl vs. now) though most are long. They're in a class by themselves.
Happy (teaching) Reading!