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 primarily affects the way the definite article (equivalent to "the" in English) is assimilated or pronounced before certain consonants.

For example, when a word begins with a sun letter, the definite article assimilates with the initial consonant of the word. In Maltese, the word for "the Sun" (xemx) is not pronounced or written as il-xemx but rather as ix-xemx. Similarly, in Arabic, the definite article al- undergoes a change when preceding sun letters, reflecting a phonetic blending that distinguishes these letters from their moon letter counterparts, which do not trigger such assimilation.

The names stem from how the definite article interacts with "the Sun" and "the Moon" in Arabic and Maltese. In Arabic, al-shams becomes ash-shams (assimilating the lām), while al-qamar remains unchanged. Similarly, in Maltese, "the Sun" is ix-xemx (with assimilation) and "the Moon" is il-qamar (without assimilation). This reflects the distinction between sun and moon letters in both languages.