The three dimensions and six workings of a solid
A board is a plane body of uniform thickness, worked by the same rules as a surface
Leonardo lays down the definitions behind the board problems: every body has three dimensions (width, thickness, length) and undergoes six transformations. A surface has only width and length and no thickness, whereas a board is a plane body of uniform thickness, so it may be worked and divided by the same rules as surfaces. A marginal note records how three of the cases arise from paired changes such as thickening and thinning.
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Three dimensions and the six workings of a body
Leonardo states that all bodies have three dimensions - width, thickness and length - and that their transformations ('travagliamenti') are six: shortening and lengthening, thickening and thinning, widening and narrowing. A surface has only width and length and no thickness, while a board is a plane body of uniform thickness, and so may be worked and divided by the same rules as surfaces.
Marginal note on the origin of three cases
A note in the margin lists how three of the cases arise: the first from thickening and thinning, the second from shortening and thinning, and the third from shortening and lengthening.