The Euclidean space is more than a simple vector space. To identify a figure in this space it must be possible to establish distances and angles, starting from the straight line which can be drawn between two points. For Euclides, a straight line was a primitive concept, the second fundamental object of geometry. It is the straight line in YY which makes the piece static, increases its stability, helps to identify a structure which transfers the properties of a geometric solid to the wood.