Ar is the twentieth letter of the Mandaic alphabet and writes r. It joins on both sides and takes a full set of four shapes, so it carries a connected run on rather than breaking it as the right-joining letters do.
Its name follows the alphabet's usual pattern of the sound with a preceding a. The head that stands behind Syriac rish and Hebrew resh has no counterpart in the Mandaic name.
The twentieth place is the one resh holds in the inherited Aramaic order, which the first twenty-two Mandaic letters keep unchanged.
Unlike Syriac, where a single stroke serves for both dalath and rish and a point alone tells them apart, the Mandaic shapes for ad and ar are separate signs, so the script does not carry that particular ambiguity of its Aramaic relatives. That is a real difference in how much work the reader has to do, and it is one of the few places where Mandaic is plainly easier to read than the scripts beside it. The other is the vowels, which Mandaic writes with full letters where its relatives leave them out.
In Hebrew and Syriac the letter at this place carries the numeral value two hundred, one of the four that hold the hundreds, and the Arabic abjad reckoning gives it the same. Mandaic records no numeral use for any of its letters, so the twentieth place here carries no arithmetic with it, and the run of four closing letters that does the counting in the other alphabets has no counterpart in this one.