Squares and a trapezoid between parallels
Similar triangles, squares, and a trapezoid whose sides ab and cd prove equidistant
Pairs of parallel lines are elaborated into a quadrangle with both diagonals, then into paired parallelograms. Adding verticals and diagonals produces similar triangles and squares standing between the parallels, with their diagonals drawn. A triangle enclosing a trapezoid, its vertices labeled a, b, c, d, illustrates that things equal to a third are equal to one another, so that sides ab and cd are equidistant. Marginal citations such as 'by the thirty-fourth' point to the propositions Leonardo is following.
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Squares between two parallels
Vertical and diagonal segments between two parallels generate similar triangles and, with further verticals, squares whose diagonals are drawn. A note reads 'all the squares of base…', continuing an argument about equal figures standing between the same parallels.
Trapezoid a b c d and equidistant sides
A trapezoid with its diagonals sits inside a triangle and is redrawn beside it with vertices labeled a, b, c, d. Because things equal to a third are equal to one another, sides ab and cd are shown to be equidistant (parallel).