Sun and moon letters

In Arabic and Maltese, all consonants are classified into two distinct groups known as sun letters (Arabic: حروف شمسية ḥurūf shamsiyyah, Maltese: konsonanti xemxin) and moon letters (Arabic: حروف قمرية ḥurūf qamariyyah, Maltese: konsonanti qamrin)

This distinction affects the way the definite article (equivalent to "the" in English) is assimilated or pronounced before consonants: when a word begins with a sun letter, the definite article assimilates with the initial consonant of the word.

The names stem from how the definite article interacts with the nouns "Sun" and "Moon" in Arabic (and Maltese). In Arabic, al-shams (“the Sun”) becomes ash-shams (assimilating the lām), while al-qamar (“the Moon”) remains unchanged. Similarly, in Maltese, "the Sun" is ix-xemx (with assimilation), while "the Moon" is il-qamar (without assimilation).