Compression tee 15mm x 15mm x 22mm.
22mm on the branch.
Sometimes called a "Bullnose" tee.
Made to the European EN1254 standard.
Compression plumbing fittings do not require heat or solder. The joint is formed by tightening a nut and compressing the olive (included with fitting) onto a pipe, which forms the seal. They can be used on wet pipework, and are suitable for hot and cold systems.