Vowels are produced by a continuous airstream and all are voiced (at least in English – Japanese does have voiceless vowels, however). They are classified according to height of the tongue, part of tongue involved, and position of the lips. The tongue can be high, mid, or low; and the part of the tongue used can be front, central or back.
Only four vowels are produced with rounded lips and only four vowels are considered tense instead of lax. The sound /a/ would be written as a low back lax unrounded vowel. Many languages also have vowels called diphthongs, a sequence of two sounds, vowel + glide. Examples in English include oy in boy and ow in cow. In addition, vowels can be nasalized when they occur before nasal consonants. A diacritic mark [~] is placed over the vowel to show this. The vowel sounds in bee and bean are considered different because the sound in bean is nasalized.