The Arabic definite article is written al- before every noun without exception, and it is not pronounced that way. Before fourteen of the twenty-eight letters the lam disappears in speech and the following consonant is doubled in its place. Those fourteen are called the sun letters, after the word for sun, which begins with shin and is therefore written al-shams and pronounced ash-shams. The other fourteen are the moon letters, after the word for moon, which keeps its lam and is both written and pronounced al-qamar.
The division is not arbitrary. The sun letters are the coronals, the consonants made with the tip or the blade of the tongue against the teeth or the ridge behind them, which is where lam is made as well. Assimilation of one coronal to a neighbouring coronal is ordinary in Semitic phonology, so the split records a phonetic fact rather than imposing a convention. The fourteen are ta, tha, dal, dhal, ra, zay, sin, shin, sad, dad, the emphatic ta, za, lam and nun.
The written form matters religiously as well as grammatically. Because the article is always written al-, the consonantal skeleton of the Quranic text stays stable while its recitation is governed by rules the letters do not fully record. The disciplines that transmit those rules, tajwid and the science of the variant readings, live in exactly that space between what the script shows and what the voice does. Sun and moon letters are the first and simplest instance of that space a learner meets, and the fact that a reader must know the rule rather than read it off the page is the point.