On the picture: Decorative stone carving with the representation of a theater mask, referring to their use in classical plays.

The Ancient Greek term for a mask is ‘prosopon’ (lit., "face"), and was originally a significant element in the worship of Dionysus at Athens, likely used in ceremonial rites and celebrations. The mask is known to have been used since the time of Aeschylus (1st half 5th century BC) and considered one of the iconic conventions of classical Greek theatre.
Masks were also made for members of the chorus, who play some part in the action and provide a commentary on the events in which they are caught up. Although there are twelve or fifteen members of the tragic chorus, they all wear the same mask because they are considered to be representing one character.

Correspondent: J.M.Criel, Antwerpen.
Source: Wikipedia.