Definitions of lines, angles and triangles
Geometry notes beneath a red-chalk cutaway of a domed building
Folio 11v sets out elementary geometry: a line is called indefinite when its letters are written beneath it and terminated when they stand at its head; the letter naming an angle must lie in the middle of the others; the unnamed side of a triangle is understood to be its base; and alternate and opposite angles — intrinsic and extrinsic — are labelled on a lettered figure. Small line- and angle-diagrams (a, b, c; a c, m n, o t, d) with strings of numbers accompany the text. At the top of the sheet, in red chalk, is a cutaway view of a centrally-planned domed building with a lantern, with a small squared figure below reading as its plan.
On this page
When a line is indefinite and when it is terminated
a is the beginning of the line; c is indefinite. When the letters are placed beneath the line, that line is called indefinite; but when they are set at the head of the line, that line is terminated.
Naming an angle and the base of a triangle
The letter that names an angle must always stand in the middle of the named letters. Of all the sides of triangles, the one that is not named is understood to be the base.
Alternate and opposite angles, intrinsic and extrinsic
d c e a b are called extrinsic co-alternate angles, and n t e m o intrinsic co-alternate angles; n o m t are opposite intrinsic angles, a d e c b opposite extrinsic angles, a m e c n the intrinsic-extrinsic opposite angle, and the like is said of o b e d t.
Cutaway of a centrally-planned domed building
In red chalk at the top of the sheet a building with a central cupola and lantern is shown cut open to reveal its vaulted interior; a small squared figure below reads as its plan.