Refuting the equality of coalternate angles
Interior angles taken on one side equal two right angles
Leonardo continues the theory of parallels, here refuting that the coalternate angles are equal. He draws two parallels crossed by a diagonal, marked with dots, that bend and meet toward the left. The note adds that the interior angle is not equal to the opposite exterior angle, cites 'the fifteenth' as proof, and states that the interior angles taken on one side equal two right angles.
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Interior angles on one side equal two right angles
The figure shows two parallels crossed by a diagonal that bend together to meet on the left. The note refutes the equality of the coalternate angles and denies that the interior angle equals the opposite exterior angle, citing 'the fifteenth' as proof. It concludes that the interior angles taken on one side are equal to two right angles.