Ash is the twenty-first letter of the Mandaic alphabet and writes a voiceless postalveolar sibilant, the sh of English ship. It joins only to the right, so the letter after it begins a new connected group.
With the affrication mark beneath it the letter covers affricates the inherited alphabet had no sign for, the sounds written ch and j in English spelling, which loan words bring in. The affrication mark is written beneath the letter and, like the other two diacritics, may be left off.
Twenty-first place is where shin stands in the Aramaic order. Its name is one of the irregular ones in the sense that it is built on the sound sh rather than on a bare consonant with a preceding a, though it follows the same principle as the regular names: the name says how to pronounce the letter.
Ash is one of the letters that break a word into more than one connected run, the others among the inherited signs being halqa, az, it and aksa, together with the three signs that close the alphabet. Those breaks are part of what makes joined Mandaic legible rather than one unbroken stroke, and a reader uses them to segment a line before recognising any individual letter.
In Hebrew and Syriac the twenty-first letter carries the numeral value three hundred, and the Arabic abjad reckoning gives it the same. Mandaic records no numeral use for its letters at all, so ash marks a place in the sequence and a sound in the language and nothing beyond them.