Which type of adaptor is used to connect two female couplings to each other?

The correct choice is the double-female adaptor because this type of fitting is specifically designed to connect two female couplings. In fire service operations, equipment such as hoses and fittings often come with male and female ends, which allows for a variety of connections. A double-female adaptor facilitates the connection of two hoses or pipes that both have female ends, allowing the flow of water to continue without needing to change the couplings on one of the hoses.

In the context of firefighting and hose operations, the double-female adaptor is crucial for creating longer runs of hose or for connecting apparatus that may not have a compatible coupling format. It ensures that both hoses can be effectively joined without leaks, which is vital for maintaining pressure during operations.

The other options represent different types of adaptors used for other purposes. For instance, a double-wye is used to split one water source into two separate lines, while a double-Siamese generally connects two hoses to a single outlet. A double-male adaptor connects two male couplings, which is not suitable in this situation since the question specifically asks for an adaptor that connects two female couplings.