14 To a triangle given, in a rectilineall angle given, make an equall parallelogramme.
As here to the triangle, aei, given in s, the right lined angle given, you may equall the parallelogramme ouyi.
15 A parallelogramme doth consist both of two diagonals, and complements, and gnomons.
For these three parts of a parallelogramme are much used in Geometricall workes and businesses, and therefore they are to be defined.
16 The Diagonall is a particular parallelogramme having both an angle and diagonall diameter common with the whole parallelogramme.
First the Diagonall is defined: As in the whole parallelogramme aeio, the diagonals are auys, and ylir; Because they are parts of the whole, having both the same common angles at a, and i: and diagonall diameter ai, with the whole parallelogramme: Not that the whole diagonie is common to both: But because the particular diagonies are the parts of the whole diagony. Therefore the diagonalls are two.
17 The Diagonall is like, and alike situate to the whole parallelogramme: è 24. p vj.