Asz is the eighteenth letter of the Mandaic alphabet and writes an emphatic s, the counterpart of sadhe in the other Aramaic alphabets. It joins on both sides and takes a full set of four shapes.
Its name is spelled with sz in the modern character standard to keep it apart from as, the fifteenth letter, in a naming scheme that has to work in plain Latin letters with no diacritics at all. Nothing in Mandaic corresponds to the z, and the spelling says nothing about how the letter sounds.
With the affrication mark beneath it the letter carries a further sound from outside the inherited set, one of the several the mark supplies.
Eighteenth place is the one sadhe holds in the inherited Aramaic order, which the Mandaic sequence follows without alteration as far as the twenty-second sign. In Hebrew, Syriac and the Arabic abjad reckoning the letter at that place carries the numeral value ninety and closes the run of tens. Mandaic records no numeral use for its letters, so the eighteenth place here marks nothing but a position in the sequence and a sound in the language.
Because asz joins on both sides it takes initial, medial, final and isolated shapes and keeps a connected run going. Its counterpart in Syriac, sadhe, joins only to the right. The same place in the same inherited sequence is therefore filled by letters that behave differently in the two scripts, which is a reminder that joining behaviour was settled by each script on its own and is not part of the inheritance.