Another principal cause of added prolongation is sukun. When a madd letter is followed by a sukun that belongs originally to the word or the pronounced name of a letter, rather than being created by a pause, the rule of madd lazim arises. It differs from a temporary pause-based prolongation.
Original sukun: remains when recitation is joined as well as when it stops.
Incidental sukun: emerges only because a final vowel is silenced at a pause.
The conventional exposition measures obligatory madd as three alifs. It may occur in ordinary words or in the names of disjointed opening letters. Whether the following consonant is doubled yields the further distinction between muthaqqal (heavy/doubled) and mukhaffaf (light/undoubled).