01The rule in one minute
Arabic has one word for “the”: the prefix ال, pronounced al-, joined to the front of the noun. It never changes for gender or number.
Before a moon letter (حَرْفٌ قَمَرِيٌّ) you pronounce the l: الْقَمَر, al-qamar, “the moon”. Before a sun letter (حَرْفٌ شَمْسِيٌّ) the l is not pronounced; the tongue goes straight to the next letter and holds it twice as long: الشَّمْس, ash-shams, “the sun”. The two words that gave the groups their names are themselves the model examples.
Only pronunciation changes. The l is written in both cases.
ق is a moon letter: al-qamar, with a clear l.
ش is a sun letter: ash-shams. The l is silent and the sh is held, as in “fresh sheets”.
02The 14 sun letters and the 14 moon letters
Sun letters: ت ث د ذ ر ز س ش ص ض ط ظ ل ن.
Moon letters: ء (and the alif that carries it) ب ج ح خ ع غ ف ق ك م ه و ي.
You do not need to memorise either list. Sun letters are the ones you make with the tip or the front of your tongue against your teeth or the ridge just behind them, which is exactly where l is made. The l blends into a neighbour made in the same spot. Moon letters are made somewhere else: with the lips (ب ف م و), at the back of the mouth (ق ك خ غ), in the throat (ء ع ح ه), or with the middle of the tongue (ي).
One letter breaks the pattern: ج. In fuṣḥā (Modern Standard Arabic) it is a moon letter, so الْجَمَل is al-jamal, “the camel”. Some spoken dialects assimilate it, which is why you may hear it differently from people who speak dialect.
ب is made with the lips, so it is a moon letter: al-bayt.
ن is made with the tongue tip behind the teeth, so it is a sun letter: an-nūr.
ل is a sun letter too: the article’s l merges with the word’s own l, and you hold one long l. al-layl, with a doubled l.
The exception: ج is a moon letter in fuṣḥā, so you hear the l. al-jamal.
03How it looks on the page
In fully vowelled text the marks show you what to do. Before a moon letter, the ل carries a sukūn, the small circle that means “no vowel here”: الْقَمَر. Before a sun letter, the ل carries no mark at all and the next letter carries a shadda, the small w-shaped sign that doubles a letter: الشَّمْس.
A common beginner’s mistake in writing is to put a sukūn on the ل and a shadda on the sun letter at the same time. The two contradict each other: a silent l has no sukūn.
Most printed Arabic leaves the vowel marks out, and then the article looks exactly the same in القمر and in الشمس. You decide from the first letter of the noun, which is why it pays to recognise the two groups by ear and by tongue position, not just by marks.
Sukūn on ل, no shadda on ك: al-kitāb.
No mark on ل, shadda on د: ad-dars.
04In a sentence, and in place names
The a at the start of al- is only heard when the word begins what you say. After a word that ends in a vowel, it drops out and you go straight from that vowel to the l, or to the doubled sun letter. The written alif stays on the page but is silent (the Arabic name for this dropping a is hamzat al-waṣl).
Place names are a good place to hear the rule, because Latin spellings often hide it. The capital of Saudi Arabia is written “Al Riyadh” or “Riyadh”, but in Arabic it is الرِّيَاض, ar-riyāḍ. Casablanca is الدَّارُ الْبَيْضَاءُ, “the white house”: its first word starts with a sun letter and its second with a moon letter, so in one name you hear both.
al-qalamu wa-d-daftar: after wa, the a of the article drops, and the d is doubled because د is a sun letter.
al-waladu wa-l-bint: the a drops, the l stays because ب is a moon letter.
ad-dāru l-bayḍāʾ: د is a sun letter, ب a moon letter.
05Practise it in Kalima
The definite article is part of the first stage of Kalima’s path, right after the short vowels: you read words such as الشَّمْس, الْقَمَر and النُّور and choose how each one sounds, so the sun and moon split is checked, not just explained.