The Circle as a 'Parallel' Figure; Equal Triangles on a Base
Radii meeting the circumference at right angles; rectilinear and curvilinear triangles of equal area
Leonardo calls the circle a 'parallel figure' because every radius from center to circumference is equal and meets the circumference at spherical right angles. A wavy line, a circular sector set over a smaller circle, and a triangle spiralling within a circle accompany the text. He states that rectilinear and curvilinear triangles built on the same base and uniformly varying in width are equal. The sketches are partly in faint pencil and partly inked in pen.
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The circle as a 'parallel' figure
Leonardo calls the circle a parallel figure because all straight lines drawn from the center to the circumference are equal and meet the circumferential line at equal spherical right angles. He adds that the same holds for the transverse lines of the parallel, which fall on their sides at right angles.
Equal area of rectilinear and curvilinear triangles
A triangle spiralling within a circle illustrates the claim that all pyramids (triangles), rectilinear and curvilinear, built on the same base and uniformly varying in width along their length, are equal. The figures lie between parallel circumferential lines.
A curved line and a sector over a smaller circle
A wavy line is captioned simply 'curve', and a circular sector is set over a circle of smaller radius. A note calls the construction equidistant from the center of the world.