A 
compression reducing set is a type of plumbing fitting used to connect pipes of different diameters within a compression joint system. It typically consists of a set of three pieces that allow a standard compression fitting (such as an elbow or tee) to adapt to a smaller-sized pipe without requiring soldering or heat. 
Overview
Function: A compression reducing set is an essential item for adapting standard compression plumbing fittings to accept smaller pipework. It creates a secure, mechanical, and leak-proof seal by compressing an olive (or ferrule) onto the smaller tube within the larger fitting.
Material: These sets are commonly made from durable brass, which offers excellent resistance to corrosion and ensures a long-lasting connection.
Applications: They are versatile and widely used in domestic and commercial plumbing, heating systems, and low-pressure gas installations, suitable for use with copper, stainless steel, and certain plastic pipes.
Advantage: A key benefit is that installation doesn't require soldering, making them ideal for confined spaces, retrofits, or situations where the use of a heat source is not possible. They can also be easily dismantled and rebuilt for maintenance.