In the Arabic language, the consonants are divided into two groups, called the sun letters (or solar letters) and moon letters (or lunar letters), based on whether or not they assimilate with the ﻝ of a preceding article. The word for "the sun", aš-šams, assimilates, while the word for "the moon", al-qamar, does not. It was from this circumstance that the two categories of consonant were named.
When followed by a sun letter, the Arabic definite article is not pronounced al-. Instead, the l sound of al- assimilates to the initial consonant of the following noun, resulting in a doubled (geminate) 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 14 sun letters are ﻥ ,ﻝ ,ﻅ ,ﻁ ,ﺽ ,ﺹ ,ﺵ ,ﺱ ,ﺯ ,ﺭ ,ﺫ ,ﺩ ,ﺙ ,ﺕ; transliterated from left to right , , , , , , ', , , , , , , .
The 14 moon letters are ه ,ﻱ ,ﻭ ,ﻡ ,ﻙ ,ﻕ ,ﻑ ,ﻍ ,ﻉ ,ﺥ ,ﺡ ,ﺝ ,ﺏ ,ء; transliterated from left to right , , , , , , , , , , , , , .
What the sun letters have in common is that they were coronal consonants in the classical language, while the moon letters were not. Since the article ends is a coronal consonant, it lends itself to assimilation with these sounds. The letter ج is a coronal consonant [ʒ] or [dʒ] in most varieties of Arabic today; however, is was a moon letter (palatalized voiced velar plosive, /ɡʲ/) in the classical language, and children learning Standard Arabic in school need to be taught that it therefore does not assimilate the article.