Hitherto we have spoken of certaine common and generall matters belonging unto parallelogrammes: specials doe follow in Rectangles and Obliquangles, which difference, as is aforesaid, is common to triangles and triangulates. But at this time we finde no fitter words whereby to distinguish the generals.
2. A Right angle is a parallelogramme that hath all his angles right angles.
As in aeio. And here hence you must understand by one right angle that all are right angles. For the right angle at a, is equall to the opposite angle at i, by the [10 e x].
And therefore they are both right angles, by the [14 e iij]. The other angle at e, and o, by the [4 e vj], are equall to two right angles: And they are equall betweene themselves, by the [10 e x]. Therefore all of them are right angles. Neither
may it indeed possible be, that in a parallelogramme there should be one right angle, but by and by they must be all right angles.
Therefore
3 A rightangle is comprehended of two right lines comprehending the right angle 1. d ij.
Comprehension, in this place doth signifie a certaine kind of Geometricall multiplication. For as of two numbers multiplied betweene themselves there is made a number: so of two sides (ductis) driven together, a right angle is made: And yet every right angle is not rationall, as before was manifest, at the [12. e iiij], and shall after appeare at the [9 e].
And