From DT Online

Using modern adhesives and production methods, curved lengths of timber can be created by Cold Moulding or Steam Bending when strength is needed. Historically however (and when there were rather more forests than exist today), naturally curved timbers were used. These were known as Crucks (aka Crooks) and were obtained by splitting, or cleaving, a suitably curved tree trunk, its roots, or the intersection of the trunk and a large branch.

Gateshead Garden Festival Cruck Barn Exterior

The Grain of the timber of such Grown Crucks would follow the shape required. Simply cutting a shape out of a plank of wood may result in places where the wood grain is oriented more across the width than along the length of the piece. This is known as Short Grain and is a weakness which is likely to break under load.

An additional advantage of Grown Crucks is that, by splitting down the centre of the tree, two matching halves are produced. This feature is taken advantage of in the construction of barns and other buildings based on Cruck Frames.

Crucks were also much sought after for wooden boat building where they were used for their strength to make the corner braces, or Knees, where the Deck Beams attach to the hull Frames. See On Naval Timber and Aboriculture from Project Gutenburg.