At is the twenty-second letter of the Mandaic alphabet and writes t. It joins on both sides and takes a full set of four shapes according to where it falls in a connected run, so a word ending in at closes with the final form rather than the isolated one.
With the affrication mark beneath it, at stands for a voiceless dental fricative, the th of English think.
It is the last of the twenty-two signs that answer one for one to the inherited Aramaic alphabet, and everything after it in the sequence is a Mandaic addition. The two signs that follow it, dushenna and kad, both began as ligatures of letters already in the list, which is how the alphabet grew past twenty-two without borrowing a shape from anywhere. The one sign that was borrowed, ain, came much later and from Arabic.
Its name follows the alphabet's usual pattern, and the doubled t of att, the ninth letter, exists only to keep the two apart in a transliteration scheme with no diacritics. At writes the plain t and att the emphatic one, and the doubling says nothing about length.
In Hebrew and Syriac the twenty-second letter carries the numeral value four hundred, the highest that any single letter of those alphabets holds, and the same is true in the Arabic abjad reckoning. Mandaic records no numeral use for its letters, so nothing here marks the twenty-second place as the end of a counting system. It is the end of the inherited block and nothing else.