Ag is the third letter of the Mandaic alphabet and writes g. It joins on both sides.
A mark written under a letter, called the affrication mark, extends the alphabet to sounds the language did not originally have, and ag carrying that mark stands for a voiced velar fricative, the sound written gh in transcriptions of Arabic and Persian. The mark is one of three diacritics in the script, the others marking vowel quality and gemination, and all three may be left off in ordinary writing and supplied by the reader.
Mandaic took in a large Iranian vocabulary early, and Arabic and Persian influence on the modern spoken dialect is heavy, so the marked letters are what a writer reaches for when a loan word will not fit the inherited sounds.
Ag holds the third place, where g stands in Hebrew, in Syriac and in the older Aramaic order. Its name is formed on the alphabet's usual plan, the sound with a preceding a, and carries no separate sense; the camel behind Syriac gamal and Hebrew gimel has no counterpart here.
Because ag joins on both sides it takes a full set of four shapes and carries a connected run onward, unlike the letters that join only to the right and force the run to restart after them.