A "sun letter" is a phonological concept in Arabic grammar that refers to a letter that becomes assimilated or affected when followed by certain Arabic particles beginning with a specific sound. In Arabic, there are 28 letters, and the sun letter is designated to one of those letters, namely the consonants [ت t], [ث th], [د d], [ذ dh], [ر r], [ز z], [س s], [ش sh], [ص ṣ], [ض ḍ], [ط ṭ], [ظ ẓ], [ق q], [ف f], and [ك k].
When any of these sun letters is followed by one of the Arabic particles or pronouns starting with a "shamsi" (solar) sound represented by the Arabic letter [ل l] or [ن n], a phonetic assimilation occurs. Specifically, the sun letter takes on the characteristics of the following particle, where its own sound is altered or "assimilated" into a similar sound. This occurs in order to ease the pronunciation and transition between two distinct sounds.
For example, the word "kitāb" (book) starts with the sun letter [ك k], and when combined with the definite article "al" (the), the pronunciation assimilates the [ك k] sound into a [g] sound, resulting in "al-kitāb." Similarly, the word "rajul" (man) starts with the sun letter [ر r], so when combined with the pronoun "ī" (my), the [ر r] sound changes to [gh], leading to "rāghli."
Understanding the concept of the sun letter is crucial in Arabic grammar and pronunciation as it affects the
