Arabic letters are divided into two groups based on how they interact with the definite article ال (al-): sun letters (الحروف الشمسية) and moon letters (الحروف القمرية). This classification affects pronunciation and is essential for correct Arabic reading and Tajweed.
Sun letters are those where the لام (laam) of ال is assimilated into the first letter of the word, creating a doubled (mushaddad) consonant. The sun letters are: ت, ث, د, ذ, ر, ز, س, ش, ص, ض, ط, ظ, ل, ن. For example, الشَّمس is pronounced "ash-shams" (not "al-shams"), and النَّاس is pronounced "an-naas" (not "al-naas").
Moon letters are those where the لام of ال is pronounced clearly, without assimilation. The moon letters are: أ, ب, ج, ح, خ, ع, غ, ف, ق, ك, م, ه, و, ي. For example, القَمَر is pronounced "al-qamar" (the laam is clearly heard), and الكتاب is pronounced "al-kitaab."
The names "sun" and "moon" come from the Arabic words themselves: الشمس (ash-shams, the sun) demonstrates assimilation (sun letter), while القمر (al-qamar, the moon) demonstrates clear pronunciation (moon letter). This naming convention is a convenient mnemonic used by Arabic teachers worldwide.
In written Arabic, the لام is always written in ال regardless of whether it's a sun or moon letter — the difference is purely in pronunciation. However, when a sun letter follows ال, a شدّة (shadda) appears on that letter to indicate the doubling. Mastering sun and moon letters is one of the first Tajweed topics covered on Ilm Al Lughah and forms the foundation for correct Quranic recitation.