Heavy letters (ḥurūf al-istiʿlāʾ) are pronounced by elevating the back of the tongue toward the soft palate, producing a full-mouth resonance — they require fullness and depth of sound. Seven letters are naturally heavy.

Light letters (ḥurūf al-istifāl) are pronounced with the tongue low and flat, producing a thin, clear sound with no oral rounding or fullness — the opposite of heavy letters. Most Arabic letters are naturally light.