With the BlueBuilt Y-shaped Drain Set, you can connect 2 drain hoses to the water drain at the same time. With this set, you can drain the water in 1 drain. The drain hose of your washing machine goes into the large opening, the drain hose of your dryer into the other small opening. The diameter of the hole for the drain hose of your washing machine is 40mm. The diameter of the hole for your dryer's drain hose is 20mm at the narrowest part and 45mm at the widest part.