Joinery is a part of woodworking that involves joining together pieces of wood or lumber, to produce more complex items. Some wood joints employ fasteners, bindings, or adhesives, while others use only wood elements. The characteristics of wooden joints - strength, flexibility, toughness, appearance, etc. - derive from the properties of the materials involved and the purpose of the joint. Therefore, different joinery techniques are used to meet differing requirements. For example, the joinery used to construct a house can be different from that used to make puzzle toys, although some concepts overlap. In British English usage it is distinguished from carpentry which relates to structural timber work.
This must be taken into account when joining wood parts together, otherwise the joint is destined to fail. Gluing boards with the grain running perpendicular to each other is often the reason for split boards, or broken joints. Furniture from the 18th century, while made by master craftsmen, did not take this into account. The result is this masterful work suffers from broken bracket feet, which was often attached with a glue block which ran perpendicular to the base pieces. The glue blocks were fastened with both glue and nails, resulting in unequal expansion and contraction between the pieces. This was also the cause of splitting of wide boards, which were commonly used during that period.
In modern woodworking it is even more critical, as heating and air conditioning cause major changes in the moisture content of the wood. All woodworking joints must take these changes into account, and allow for the resulting movement.
Wood is stronger when stressed along the grain (longitudinally) than it is when stressed across the grain (radially and tangentially). Wood is a natural composite material; parallel strands of cellulose fibers are held together by a lignin binder. These long chains of fibers make the wood exceptionally strong by resisting stress and spreading the load over the length of the board. Furthermore, cellulose is tougher than lignin, a fact demonstrated by the relative ease with which wood can be split along the grain compared to across it.
Timber expands and contracts in response to humidity, usually much less so longitudinally than in the radial and tangential directions. As tracheophytes, trees have lignified tissues which transport resources such as water, minerals and photosynthetic products up and down the plant. While lumber from a harvested tree is no longer alive, these tissues still absorb and expel water causing swelling and shrinkage of the wood in kind with change in humidity. When the dimensional stability of the wood is paramount, quarter-sawn or rift-sawn lumber is preferred because its grain pattern is consistent and thus reacts less to humidity.
Many traditional wood joinery techniques use the distinctive material properties of wood, often without resorting to mechanical fasteners or adhesives. While every culture in which pieces of wood are joined together to make furniture or structures has a joinery tradition, wood joinery techniques have been especially well-documented, and are celebrated, in the Indian, Chinese, European, and Japanese traditions. Because of the physical existence of Indian and Egyptian examples, we know that furniture from the first several dynasties show the use of complex joints, like the Dovetail, over 5,000 years ago. This tradition continued to other later Western styles. The 18th century writer Diderot included over 90 detailed illustrations of wood joints in his comprehensive encyclopedia. While Western techniques focused on concealment of joinery, the Eastern societies, though later, did not attempt to "hide" their joints. The Japanese and Chinese traditions in particular required the use of hundreds of types of joints. The reason was that nails and glues used did not stand up well to the vastly fluctuating temperatures and humid weather conditions in most of Central and South-East Asia. As well, the highly resinous woods used in traditional Chinese furniture do not glue well, even if they are cleaned with solvents and attached using modern glues.
Methods that are not considered traditional joinery have come about in modern times, largely to attempt to simplify the job of the woodworker for various reasons. These include biscuit joints and pocket hole joinery.