<P> 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> Different species of wood have different strength levels, and the exact strength may vary from sample to sample . </P>

How many types of wood joints are there