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The Moon and Sun letters in Arabic refer to the letters which when combined with the Arabic definite article ( ال ) either make the ل sound before it silent 🌞 , as done by Sun letters; or have no effect on it 🌙 , as it is the case with Moon letters.

The Moon letters or “Huruf qamariyah” (الحروف القمرية ) are the letters that have no impact on the definite article. The ل /lam/ is always pronounced when it’s followed by a moon letter.

The Sun letters, however, do have an impact on the ل before it by making it silent. So, basically the Sun letters are the ones to watch out for.