Proposition: 3.16: Line at Right Angles to Diameter of Circle At its Ends Touches the Circle

(Proposition 16 from Book 3 of Euclid's “Elements”)

A (straight line) drawn at right angles to the diameter of a circle, from its end, will fall outside the circle. And another straight line cannot be inserted into the space between the (aforementioned) straight line and the circumference. And the angle of the semicircle is greater than any acute rectilinear angle whatsoever, and the remaining (angle is) less (than any acute rectilinear angle).


Modern Formulation

A straight line ($AE$) going through the endpoint ($A$) of a diameter ($\overline{AB}$) of a given circle is perpendicular to this diameter and lies outside the circle.


Then for any acute rectilinear angle $\gamma$ the following holds: $\alpha < \gamma < \beta.$

Proofs: 1 Corollaries: 1

Proofs: 1 2 3 4

Thank you to the contributors under CC BY-SA 4.0!



Adapted from (subject to copyright, with kind permission)

  1. Fitzpatrick, Richard: Euclid's "Elements of Geometry"

Adapted from CC BY-SA 3.0 Sources:

  1. Prime.mover and others: "Pr∞fWiki",, 2016


  1. Please note that the angles $\alpha$ and $\beta$ are not rectilinear but formed between a straight line (a tangent respectively a diameter) and a curved line (the circumference of the circle).