Consonants are divided into two groups: stops and semi-vowels. The more pure consonants are given the label Stops and the less pure consonants given the label Semi-Vowels. A stop requires a full restriction of air before the sound is made.

Pronounce the letters 'p' (a stop), 'm' (a semi-vowel) and 'e' a vowel. It will be clear why the term 'Stop' is used for a letter like 'p.' Air must be restricted (or stopped) before the sound can be created, unlike the letters called Semi-vowel and Vowels.