Triangles on the same base are proportional to their heights
Triangles a b c and d b c on base b c, one double the other
The page states a Euclidean theorem: triangles standing on the same base have the same ratio to one another as their heights. Leonardo illustrates with the triangles a b c and d b c built on base b c, so that when a b is double d b, triangle a b c is double triangle d b c. The diagram at the top shows a tall right triangle standing on a vertical side, with an internal line (d) dividing it, drawn between the parallels a b and d c.
On this page
Triangles on one base proportional to their heights
Triangles placed on the same base have between them the same proportion as their heights. With triangles a b c and d b c on base b c, since a b is double d b, triangle a b c is double triangle d b c. The proof appeals to a prior proposition on equal bases and heights set between the parallels a b and d c.
Ratio of areas equals ratio of heights
The ratio between the two triangles is identical to the ratio between their heights. A doubling of the height a b over d b produces a doubling of the corresponding area.