Sad is the fourteenth letter of the modern Arabic order and the eighteenth of the older abjad order, from which it takes the numeral value ninety. It is written as a long flat loop running into a shallow bowl that falls below the line, and it joins on both sides, keeping the loop alone at the beginning and in the middle of a word. The same skeleton carrying a single dot above is dad, one of the six letters Arabic added to the alphabet it inherited.
Sad is one of the four emphatic consonants, with dad, the emphatic ta and za. Emphasis in Arabic is a secondary articulation: the back of the tongue is raised toward the soft palate while the consonant is made, and the effect spreads to the vowels on either side, so a syllable containing sad sounds darker throughout than one containing sin. The script marks the difference by using a separate letter rather than a diacritic, which is why sad and sin look nothing alike despite writing sounds that differ only in that one quality.
Sad is a sun letter. Its name descends from the Northwest Semitic tsade, the cognate of Hebrew tzadi and Syriac sadhe, whose sense has been reconstructed as a fishhook, and it holds the same numeral value of ninety. It is among the fourteen letters that appear disjoined at the head of a sura, and it is one of the few that stands alone there rather than in a group.