Nōmen (能面 Nō masks) or Omote (面 masks) are an integral and distinguishing aspect of Nō theater. With the exception of those portraying of women and old men, Omote are not worn by all actors, and necessarily represent a supernatural, otherworldly character.
The Ko-Omote (こ面 little mask) represents a woman no older than twenty. Without tilting, the mask retains a neutral expression, but appears to become happy or sad, when the actor tilts their head downward or upward respectively.
The image above (left) shows an actor wearing a Ko-Omote. Traditionally, all actors in Nō theater were male. The use of masks enabled an otherwise homogenous dramatis personae to represent a wide range of characters and creatures.