Ductwork Connectors
Sheet metal ductwork for conveying air in HVAC systems typically uses gendered connections. Typically, the airflow through a ductwork connection is from male to female. However, since one-way flow is implemented by forced-air fans or blowers, "backwards" gendered connections can be seen frequently in some systems, since all connections are typically sealed with duct sealing mastic or tape to prevent leakage anyway. The flow convention is usually loosely adhered to for simplicity of design, and to reduce the number of gender changer fittings required, but exceptions are made whenever expedient.
Read more about this topic: Gender Of Connectors And Fasteners