Onna (meaning “woman”) masks are perhaps the most prolific Noh masks.
They are able to produce different expressions depending on their orientation which allows them to produce long complex performances. Onna masks are
intentionally made with a sort of neutral face that makes it function as an “intermediate expression mask”.
Some Okina and Otoko masks are made like this too but it’s a central feature to Onna masks.
Various masks are needed to express different ages and styles, so many masks have been made.