When the missed prayers he owes reach six—whether the whole set is old backlog (qadīm) or all recent (ḥadīth), or a mixture—the strict chronological order (tarīb) normally binding between missed prayers is lifted for that set: he may begin with whichever of the six he chooses, subject to the usual conditions of intention and validity. Once he has completed qaḍāʾ of one full prayer from those six, tarīb returns for whatever remains: thereafter he must observe sequence among the outstanding makeups. See: al-Hidāyah (Ṣalāh, qaḍāʾ al-fawāʾit); Ibn Nujaym, al-Baḥr al-Rāʾiq; Ibn ʿĀbidīn, Radd al-Muḥtār (bab al-fāʾitah).