Att is the ninth letter of the Mandaic alphabet and writes an emphatic t, one of the consonants that Semitic languages set apart from their plain counterparts by a secondary articulation further back in the mouth. It joins on both sides and takes a full set of four shapes according to where it falls in a run.
With the affrication mark beneath it the letter carries a further sound from outside the inherited set.
Att sits at the ninth place, where Aramaic has teth. The doubled t of its modern name keeps it apart in transliteration from at, the twenty-second letter, which writes the plain t. That doubling is a convention of the naming scheme rather than a fact about the letter: it says nothing about length or gemination, and the Mandaic script has a separate mark for gemination which is written with the letter and may be omitted.
The letters that join on both sides, att among them, carry a connected run onward, while the letters that join only to the right force the next letter to start a fresh one. Mandaic keeps the emphatic and plain pairs of its inherited alphabet in spelling in the same way its relatives do, so a word takes att or it takes at and the choice belongs to the root.