Lam is the twenty-third letter of the modern Arabic order and the twelfth of the older abjad order, from which it takes the numeral value thirty. Standing alone it is a tall upright that swings into a deep bowl below the line; joined, it reduces to a bare vertical stroke resembling an alif but connected on both sides. It is one of the few letters whose isolated and medial forms are so different that a learner may not connect them.
Lam is a sun letter, and it holds a position no other letter holds, because lam is the consonant of the definite article. Every noun in Arabic that is definite carries an alif and a lam in front of it, and whether that lam is pronounced or assimilated to the following consonant is what divides the alphabet into its two halves. The rule is a rule about lam, and the fourteen sun letters are simply the letters that lam gives way to.
Followed by alif, lam forms the one obligatory ligature of the script, a shape distinct enough that it is often counted as a twenty-ninth letter in teaching charts.
Its name descends from the Northwest Semitic lamed, the cognate of Hebrew lamed and Syriac lamadh, whose sense was reconstructed as an ox-goad, and it holds the numeral value thirty that Hebrew lamed holds. Lam is one of the fourteen letters that appear disjoined at the head of a sura, and it occurs in more of those groups than almost any other letter.