OA, OfA zy; MA dy, d; later d but also dy in various positions; Sam. often אד ; in Mandaic written with a unique ligature developed from "d" + "y", in our code transcribed as "ḏ" (in Roman as "D"), and considered to be the last letter of the alphabet.

Note that in PTA the full form די is almost exclusively reserved for the conjunction (with only 17 examples as a preposition) whereas the proclitic form may be either prepositional or conjunctional.
As a replacement for the construct genitive, the usage is extremely rare and clearly context driven in the older dialects.