4 Foure right angles doe fill a place.
Neither is it any matter at all whether the foure rectangles be equall, or unequall; equilaters, or unequilaters; homogeneals, or heterogenealls. For which way so ever they be turned, the angles shall be right angles: And therefore they shall fill a place.
5 If the diameter doe cut the side of a right angle into two aquall parts, it doth cut it perpendicularly: And contrariwise.
As here appeareth by the [1 e vij]. by drawing of the diagonies of the bisegments. The converse is manifest, by the [2 e vij]. and 17. e vij.
Therefore
6 If an inscribed right line doe perpendicularly cut the side of the right angle into two equall parts, it is the diameter.
The reason is, because it doth cut the parallelogramme into two equall portions.