A 3/4" x 3/4" x 15mm washing machine tee (also known as a dual appliance tee valve) is a plumbing fitting used to connect two appliances, such as a washing machine and a dishwasher, to a single 15mm water supply pipe. 
Key Features and Specifications
Function: Splits a single water line to supply two appliances simultaneously.
Connection Types:
Inlet: 15mm compression fitting, designed to connect to standard 15mm copper or plastic pipe (with an insert).
Outlets: Two 3/4" BSP male threaded connections, which is the standard size for most washing machine and dishwasher fill hoses.
Material: Typically made from durable brass, often with a chrome or nickel-plated finish for corrosion resistance.
Isolation Valves: Many models include individual quarter-turn isolation valves (taps/levers) for each 3/4" outlet, allowing you to control the water flow to each appliance independently without shutting off the main supply.
Levers: Often supplied with interchangeable red and blue handles to indicate hot and cold water lines.
Installation: Uses compression fittings for a secure, flame-free installation, making it suitable for DIY or professional use.
WRAS Approval: WRAS (Water Regulations Advisory Scheme) approved versions are available, ensuring compliance with UK water quality and safety standards. 
Common Uses
Connecting a washing machine and a dishwasher off the same water feed.
Connecting an American-style fridge freezer (for the ice/water dispenser) and another appliance.
Any multi-appliance setup in kitchens or utility rooms where water supply points are limited.