This is a sort of Tee which has the department at a 45° angle, or an angle aside from 90°. Wye tee will allow a person pipe being joined to another at a 45° angle. Such a tee lowers friction and turbulence that might hamper the flow. Wye tee is http://andrecvmcr.ttblogs.com/96051/valves-and-fittings-options