Dad is the fifteenth letter of the modern Arabic order, written as the sad skeleton with a single dot above: a long flat loop running into a shallow bowl that drops below the line. It joins on both sides and keeps the loop alone at the beginning and in the middle of a word. It is one of the four emphatic consonants, made with the back of the tongue raised toward the soft palate, and the emphasis colours the vowels on either side of it. It is a sun letter, so the lam of the definite article assimilates to it in speech.
Like tha, kha, dhal, za and ghayn, dad is one of the six letters Arabic added to the twenty-two letter sequence it inherited, and its position records that. In the modern order, arranged by shape, it stands beside the sad it is built from. In the older abjad order it is among the six appended after the inherited letters, where it carries the numeral value eight hundred. It has no Northwest Semitic name behind it and no cognate letter in Hebrew, Syriac or Samaritan whose name may be set beside it.
No letter carries more identity than this one. Arabic is called the language of dad, and Arabic speakers are called the people of dad, on the understanding that the sound is unusual among the languages of the world and that mastering it marks a speaker. The phrase is old, it is still current, and it is used as often in cultural argument as in linguistic description.