Dushenna, also called adu, is the twenty-third letter of the Mandaic alphabet. It joins only to the right.
It began as a ligature of ad and aksa, the fourth and tenth letters, whose joined form was read di and which Aramaic used to write the relative pronoun and the marker of the genitive. Mandaic treats the ligature as a letter in its own right, undividable, usually written at the head of a word or standing alone.
The ordinary joined sequence of ad and aksa still exists beside it with the same origin and a different job, which is the sort of split that happens when a shorthand hardens into a letter. The two have to be told apart by shape rather than by the parts they are made of.
Dushenna is the twenty-third sign of the abagada in the traditional count, which then closes by writing halqa a second time to reach twenty-four. It is therefore the last new sign the tradition's own reckoning names, and everything after it, kad and ain, sits outside that count and is present in the modern character standard because manuscripts require it.
Because dushenna joins only to the right, whatever follows it begins a fresh connected group, which suits a sign usually written at the head of a word or standing alone. Its other name, adu, records the two letters it was made from, and both names are current in the reference literature, so a reader meeting one should know to look for the other.