An outflow valve can be a hinged single rectangular flap or a double flap (with a common drive system).
Some aircraft also have an "overboard valve" or another outflow valve at the front of the aircraft to help balance the airflow in the cabin (you don't want all the air rushing to the back of the aircraft to the regular outflow valve/s ... as this airflow can annoy passengers). These devices can be butterfly valves or flap-type valves.
Rgds.
NSEU