The general rule of loaded cords proved on the triangle
Base, side and perpendicular of the angle; force-to-weight read as line ratios in a semicircle
Folio 78r states the 'tested proposition' behind the loaded-cord method: a weight hung from a cord bears equally along its whole length, so Leonardo makes the two cords equal (3 f and f 5) and takes their perpendicular m f to measure the force. Two triangles inscribed in semicircles, labelled base, side (cossta), interpreter and perpendicular-of-the-angle, serve as the 'definition and proof'. He reads every partial load off a ratio of segments (a b : b c, b d : d c, a e : a f, giving c the fraction 5/6), states that force is to weight as line d o is to o c, and warns that where the supports greatly exceed the equality line the cord would break under the load.
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A cord bears its load equally along its whole length
Because the weight supported by a cord is entire throughout the whole length of the cord and in every part of it, the cord a f is everywhere equally loaded by the weight of 6, and so is the cord n f. To measure the force alike for both cords Leonardo makes them of equal length, 3 f and f 5, and takes the perpendicular m f.
Base, side and perpendicular of the angle
The triangle is labelled with its base (basa), its side (cossta), the interpreter (interprete) and the perpendicular of the angle. These named parts frame the 'definition with the general rule and the proof of the triangles' that the folio sets out.
Every partial load read from a ratio of segments
The proportion a b to b c gives the weight c to weight 3a; b d to d c gives d to the combined a c; a e to a f gives c to d, so c holds 5/6; and force is to weight as line d o is to line o c. The statics reduces entirely to comparisons of lengths.
When the cord would break
The proportion n m has to m S is the weakness of m S to the force n m, a weakness restored by the power of the cord n S. When the supports far exceed the line n f in reaching the equality line n S the excess can be an infinite proportion, so the cord n S would break under the weight, being of the same nature.