At last night's performance of Donizetti's Lucia di Lammermoor at San Francisco Opera, my friend pointed to the gilded comedy and tragedy masks decorating the proscenium above the stage and asked me what their names were. She was sure she had learned their proper Grecian names in the past, but had forgotten them.
Thalia is the muse of comedy, Melpomene the muse of tragedy. Sometimes the comic and tragic masks are called by the names of these muses, sometimes not.