Ad is the fourth letter of the Mandaic alphabet and writes d. It joins on both sides. With the affrication mark written beneath it, ad stands for a voiced dental fricative, the th of English this.
Ad is also the first element of the twenty-third sign of the alphabet, dushenna, which began as a ligature of ad and aksa and hardened into a letter in its own right. The ordinary joined sequence of ad and aksa still exists beside it and is read as the syllable di, so the two have to be told apart by shape rather than by the parts they are made of.
The affrication mark is written under the letter and may be left off in ordinary text, so a reader supplies the distinction from the word rather than from the page.
Ad holds the fourth place, where d stands in the inherited Aramaic order. Unlike Syriac, where dalath and rish share a single stroke and a point alone tells them apart, the Mandaic shapes for ad and ar are separate signs, so this script does not carry that particular ambiguity of its Aramaic relatives. That is worth stating because the ambiguity is a real burden on Syriac editors, and Mandaic simply does not have it.
In Hebrew and Syriac the letter at this place carries the numeral value four, and in Syriac it is also one of the six letters that take a hard and a soft reading governed by the grammar of the word. Mandaic has neither the numerals nor that alternation. Two scripts do quite different amounts of work with the letter standing at the same place in the same inherited sequence.