The “wye” is a connection that will split one hose into two different lines. These might be needed for long driveways where the fire engine can’t maneuver, houses set far back, or a multi-family structure with limited access. One type of these is referred to as a “water thief”, or a gated wye. This can be used to allow water to only go through one side of the connection or both at the same time. These usually have a 2 1/2 inch inlet and 2 1/2 inch outlets.
There is also the siamese coupling that will turn two smaller lines into one hose line. This is commonly used to avoid the loss of friction in long hose lays. It also adds additional lines on the fireground. These usually have 2 or 3 “female” connections coming into the appliance and one “male” connection for the discharge.
There’s also the Z-adapter, which is for connecting supplemental pumps into long hose lines. These are usually adapted from two gated wyes and a double female connection. Or a siamese may be connected to one outlet of the gated wye.