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) 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, doesn't. This also applies to the Maltese language where they are written as ix-xemx and il-qamar.