A pattress or pattress box is the container for the space behind electrical fittings such as power outlet sockets and light switches.
A pattress may be made of plastic (usually white in the United Kingdom, though there is no requirement that this be so), or metal. It is usually of standard dimensions as per the relevant standards and/or regulations, and may contain embedded bushings (in standard positions) for the fixing of the front plate containing the actual switch or socket(s).
The pattress box may be designed for either surface mounting (cabling running along the wall surface) or embedded in the wall (or skirting board); in the latter case, thin unfinished galvanised metal is more common.
Amongst electricians the term "pattress" is used to describe a surface mounted box, and "wall box" for those that go inside the wall. Within brick/block walls these are usually metal. Within timber+plasterboard walls they are usually "plasterboard boxes".
Although cables may be joined inside pattress boxes, due simply to their presence at convenient points in the wiring, their main purpose is to allow the majority of the physical body of the switch or socket to protrude less from the surface, especially for the flush mounting case, for a more pleasing appearance, and also to isolate live terminals from the user. For enclosures whose primary purpose is the joining of cables/wires out of sight, see junction box.