The Forum of Trajan was enclosed by a curved wall on its southern perimeter broken by a central arch flanked by two smaller lateral ones, all of which were aligned with the porch of the Basilica Ulpia and duplicated its façade. Numismatic evidence, such as the reverse of this Trajanic aureus suggests the appearance of the monumental arch, although there are minor variations in how it is depicted.
Flanking either side of the central fornix or archway were two bays, each of which contained an aedicula (a small shrine) holding a statue framed by columns and capped by a triangular pediment. Above each was a medallion or possibly imago clipeata, a portrait bust in a circular ornamental frame, that recalled those decorating the colonnades in the forum. The columns are shown on a low stepped platform, which would have allowed only foot traffic in the forum. The thick bases suggest to Packer the pedestals on which the lower order of columns stood between the stairs in the west library and may have looked the same. The arch has a deep attic, which possibly carried a long inscription, surmounted by a seiugis (six-horse chariot) led by soldiers on either side, and flanked by trophies and Victories. Trajan is portrayed accompanied by Victory, as well.
The coin was struck in AD 115, after the dedication of the forum, itself, to celebrate Trajan's vicennalia (twentieth anniversary). Dio's remark that "the people in Rome were preparing for him a triumphal arch besides many other tributes in his own forum" (LXVIII.29.3) (in recognition of the capture of Ctesiphon, the capital of the Parthian empire) may mean, therefore, that the arch was remodeled rather than erected by the Senate in AD 117, the year of Trajan's death.
References: The Monuments of Ancient Rome as Coin Types (1989) by Philip V. Hill. The image is from the Triton III catalog (1999), item 1066; see RIC II 256, BMC 509.
