Za is the seventeenth letter of the modern Arabic order, written as the emphatic ta skeleton with a single dot placed above the loop. Like that letter it holds its shape in every position, and it joins on both sides. It is a sun letter, and one of the four emphatic consonants.
Za is one of the six letters Arabic added to the sequence it inherited, so it stands near the end of the abjad order rather than within it, carrying the numeral value nine hundred. Only ghayn is higher. That placement has a straightforward explanation: values were assigned by position in the inherited twenty-two letter sequence, the six additions were appended after it, and the last to be appended took the largest numbers. It has no Northwest Semitic name behind it.
Its sound is the emphatic counterpart of dhal, a voiced dental fricative pronounced with the back of the tongue raised. In the great majority of spoken Arabic dialects that sound no longer exists as a separate item: it has fallen together with dad in some regions and with the emphatic form of other consonants elsewhere, so that the two letters are pronounced alike and the spelling of a word must be learned rather than heard. Formal reading, and recitation in particular, holds the distinction, which is one of the reasons the recitation disciplines are taught rather than assumed.