You are Mizzen the origin of the word porthole from wikipeadia

For heavy weather and when the cannons were not in use, the openings were fitted with covers, that were called porte in French, meaning "door". "Porte" was Anglicized to "port" and later corrupted to porthole. Eventually, it came to mean any opening in a ship's side whether for cannon or not.

Please do not ask me why port became also used to designate ONE side of a ship AND a drink , but I am sure that you can find those.

Doors are needed (for light and access) on both sides of a ship

Yo is mizzen a mainmast.