Arabic Mathematical Initial Ṣād

It comes from ص, but the math form stands alone. Ṣād (ص) is an Arabic letter with an emphatic s sound. “Initial” names the math form, not a word position. Ordinary ص changes shape to join its neighbors; this math form does not.

Arabic mathematical handbooks use six letter styles—isolated, initial, tailed, stretched, looped and double-struck—in manuscripts and traditional print over many centuries. These distinctions matter in Arabic- and Persian-script math, including handbooks from West Asia, Libya and Algeria; the expression gives each letter its role.