Arabic (ركن) for one of the pillars/ elements/ cornerstones of a contract (aqd– عقد). The primary arkan (pl. of rukn) of a contract of sale (aqd al-ba’i/ aqd al-bay’– عقد البيع), according to the majority of fuqaha (jumhur– الجمهور), are: the two parties (seller and buyer), the language (formula/ sighat al-aqd) of the contract, and the subject-matter (object of sale/ mahall and price/ thaman). Sighat al-aqd constitutes ‘ardh/ ijab (offer) and qubul/ qabool/ qabul (acceptance). For the Hanafis (الحنفية), the essential cornerstone of a sale is ijab wa qubul (offer and acceptance), i.e., the statement signifying the willingness and acceptance to enter into a contractual agreement.

Arkan al-aqd are almost similar for exchange-based contracts (uqud al-mu’awadhah– عقودالمعاوضة).