Cruck barns evolved in the Anglo Saxon period but came into their own during medieval times.
Each frame, or cruck, was constructed of curved timbers (called blades) using the natural shape of a tree. It was the norm for a tree to be sliced long-ways down the middle so that whatever the shape of the curve the two sides would be symmetrical. The two beams were joined together at the top by a ‘collar’ or tie-beam.
There is more about cruck barns on Wikipedia
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