In short
Parthian coins are the silver drachms and large tetradrachms of the Arsacid empire (247 BCE–224 CE): a diademed or tiara-wearing king on the obverse and a seated archer holding a bow on the reverse, with Greek legends that grow progressively more blundered over five centuries.
The two main denominations
The everyday coin is the silver drachm of roughly 3.5–4 g, struck in huge numbers at Ecbatana (Hamadan) and Rhagae. The prestige coin is the tetradrachm of 14–16 g, struck almost exclusively at Seleucia on the Tigris and often carrying a Seleucid-era date and month name — the only reliable absolute dates in the series.
Bronze issues circulated locally and are usually attributed by the same portrait and reverse conventions.
How to identify a Parthian king
Attribution rests on headgear, beard and hairstyle rather than legend, since the Greek inscriptions become increasingly formulaic and eventually illegible. Early kings wear a soft bashlyk cap; Mithradates II introduces the tall domed tiara; later rulers wear a diadem with tiered hair and prominent wart-like facial features that are diagnostic of individual reigns.
The reverse archer sits on an omphalos or throne depending on period, and the arrangement of the legend's lines is itself a dating tool. Sellwood's type numbers are the standard reference and are quoted by every dealer.
Collecting and value
Common drachms of Mithradates II, Orodes II and Vologases are affordable in decent grade and make an easy entry into the series. Tetradrachms are scarcer, larger and more expensive; early Arsacid drachms and rare mints command real premiums. Because dates appear only on the Seleucia tetradrachms, a dated coin is always worth more to a specialist.