What does frontality mean in Parthian sculpture?

In Parthian sculpture, frontality refers to a stylistic tendency where figures ignore one another to face the spectator directly with an unblinking stare. This design preference is prominently observed in rock reliefs found at sites such as Tang-e Sarwak and Bīsutūn.1

This characteristic was not unique to the Parthians; it also defined the Romano-Syrian sculptures of Palmyra and eventually influenced the development of Byzantine art. In later periods, such as the Sāsānian era, this convention was largely abandoned in favor of depictions where individuals were shown in three-quarter profile.12