Sun and moon letters

In Arabic and Maltese, the consonants are divided into two groups, called the sun letters or solar letters (Arabic: حروف شمسية‎‎ ḥurūf shamsīyah) and moon letters or lunar letters (حروف قمرية ḥurūf qamarīyah), based on whether they assimilate the letter lām (ﻝ l) of a preceding definite article al- (الـ). These names come from the fact that the word for 'the sun', al-shams, pronounced ash-shams, assimilates the lām, while the word for 'the moon', al-qamar, does not.

When followed by a sun letter, the /l/ of the Arabic definite article al- assimilates to the initial consonant of the following noun, resulting in a doubled consonant. For example, for "the Nile", one does not say al-Nīl, but an-Nīl. When the definite article is followed by a moon letter, no assimilation takes place. The sun letters represent the coronal consonants according to the phonology of Classical Arabic, and the moon letters represent all others.

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