This coin was minted in Antioch. The Γ between Jupiter and Tacitus on the reverse indicates which workshop (often referred to by the Latin term officina) in the mint produced this coin. For much, if not all, of the imperial period mints were divided into different workshops which produced coinage simultaneously. In the earlier years of the imperial period, there seems to be little if any sign of the workshop present on the coins themselves, but in this later period, often the mark of the workshop is prominently shown, typically with a Greek letter, as on this coin, if the coin was eastern. The XXI in exergue is mark of purity. It serves as an official guarantee of sorts that this coin is comprised of 5% (1/20th) silver. This was particularly important because, in the years before the production of this coin, silver content had fallen as low as 2%. Jonathan Gress 2011.