In learning an easy, beneficial and (mostly) universal singing technique we work a lot with the so-called “pure” vowel. There are five, by linguists and others called the “long” vowels.
Because of the long history of teaching singing in Europe, our “pure” vowel system is based on the five long Italian vowels:

 -A (‘father’)
 -E (‘say’, but without the diphthong)
 -I (‘tree’)
 -O (‘go’, but without the diphthong) and
 -U (‘truth’)
(Italian – and the Italian form of Latin – has only two others, expressed in English as ‘eh’ and ‘aw’.)

Even though these five long vowels are also basic to the English language, it is difficult, and in some cases impossible, to write down the sound of these vowels in English.
Part of the difficulty is the existence of the diphthongs on ‘E’ (‘ay’) and ‘O’ (‘ou’) in English. We can’t do without them in English and in singing in English, but we want to start our perfecting of the open vowel technique by understanding exactly how we sing the diphthongs.

Diphthongs

A diphthong is two vowel sounds, one after the other. Usually, the first vowel – the ‘syllabic’ vowel – is long, and the second – the ‘vanishing’ vowel – is, as you would expect, very short. In singing, the first vowel is extended until the very last moment and the vanishing vowel slipped in just before changing the syllable for the next one.
In English there are five diphthongs:

– AI (‘light’, ‘time’, ‘silent’)
– AO (‘cow’, ‘plough’, ‘thou’)
– OI (‘boy’, ‘toy’)
– EI (‘say’, ‘take’, ‘tail’, ‘pale’) and
– OU (‘though’, ‘snow’)

Therefore, because the diphthongs are special cases, and we don’t consider them to be ‘pure’, in these articles on vocal technique I’ve used the Italian capital letter, in bold, for the five long – ‘pure’ – vowel sounds, A, E I, O and U. Other systems of transcribing sounds are more complicated and take longer to learn – the International Phonetic Alphabet (IPA), for instance.