In Arabic and Maltese, the consonants are divided into two groups, called the sun letters or solar letters (Arabic: حروف شمسية ḥurūf shamsiyyah, Maltese: konsonanti xemxin) and moon letters or lunar letters (Arabic: حروف قمرية ḥurūf qamariyyah, Maltese: konsonanti qamrin), based on whether they assimilate the letter lām (ﻝ l)^[1] of a preceding Arabic definite article al- (الـ), which is an important general rule used in Arabic grammar. Phonetically, sun letters are ones pronounced as coronal consonants, and moon letters are ones pronounced as other consonants.

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. This also applies to the Maltese language where they are written as ix-xemx and il-qamar.

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, "the Nile" is pronounced an-Nīl, not al-Nīl.

When the Arabic definite article (الْـ) 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. The sun and moon letters are as follows:

Hamza is not considered a letter.

The sun (konsonanti xemxin) and moon (konsonanti qamrin) letters are as follows:

If a word starts with any of the moon letters, the definite article il- stays the same and does not assimilate, while with the sun letters it assimilates accordingly to: iċ-, id-, in-, ir-, is-, it-, ix-, iż-, iz-. It is also worth mentioning that words starting with vowels, and the letters għ, and h get the definite article l- (minus the initial i). When the definite article comes exactly after a word ending in a vowel, the initial <i> of the article always drops, as in "dak ir-raġel ra r-raġel" (that man saw the man). When a word starts with two consonants, the definite article used is l-, but an i is attached at the beginning of the word: skola > l-iskola and Żvezja > l-Iżvezja.^[4]

The sound /l/ (represented by the letters L and ل) is considered a moon consonant in Maltese and a sun consonant in Arabic but they both function in the same way e.g. (the meat) is il-laħam in Maltese and الْلَحْمْ al-laḥm in Arabic or (the game) is il-logħba in Maltese and الْلُعْبَة al-luʿba in Arabic.

In the written language, the ⟨ال‎⟩ al is retained regardless of how it is pronounced.^[1] When full diacritics are used, assimilation may be expressed by putting a shaddah ⟨ ّ⟩ on the consonant after the lām ⟨ل‎⟩. Non-assimilation may be expressed by placing a sukūn over the lām ⟨لْ‎⟩.

Most modern-written Arabic names (including personal names and geographical Arabic names) do not follow the consonant assimilation rule or the shaddah when Latinized in Latin-spelled languages. Sometimes the sun and moon rules are not followed in casual speech. They are also mostly spaced rather than hyphenated.

E.g. personal name:

• الرَّحْمَن‎ - Al Rahman or El Rahman instead of Ar-Raḥmān

transliterated geographical name:

• الْجُمْهُورِيَّةُ التُّونِسِيَّة‎ - Al Jumhuriyah Al Tunisiyah instead of al-Jumhūrīyatu t-Tūnisīyah

• Arabic phonology
• Arabic grammar