Sun and Moon Letters - Rule

Rule

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.

Sun letters represent all coronal consonants except ج ǧīm (see below). Since the article al- ends in a coronal consonant, it lends itself to assimilation with these sounds.

The sun and moon letters are as follows:

Sun letters ﺕ ﺙ ﺩ ﺫ ﺭ ﺯ ﺱ ﺵ ﺹ ﺽ ﻁ ﻅ ﻝ ﻥ
t ṯ d ḏ r z s š ṣ ḍ ṭ ẓ l n
Moon letters ء ﺏ ﺝ ﺡ ﺥ ﻉ ﻍ ﻑ ﻕ ﻙ ﻡ ﻭ ﻱ ه
ʾ b ǧ ḥ ḫ ʿ ġ f q k m w y h

Read more about this topic:  Sun And Moon Letters

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