Proposition: 1.11: Constructing a Perpendicular Segment to a Straight Line From a Given Point On the Straight Line
Euclid's Formulation
To draw a straight line at right angles to a given straight line from a given point on it.
* Let $AB$ be the given straight line, and $C$ the given point on it.
* So it is required to draw a straight line from the point $C$ at right angles to the straight line $AB$.
Modern Formulation
It is possible to construct a segment at a right angle to a given straight line $AB$ from an arbitrary point $C$ on the straight line.
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Propositions: 25 26 27
Sections: 28
Thank you to the contributors under CC BYSA 4.0!
 Github:

 nonGithub:
 @Calahan
 @Casey
 @Fitzpatrick
References
Adapted from CC BYSA 3.0 Sources:
 Callahan, Daniel: "Euclidâ€™s 'Elements' Redux" 2014
Adapted from (Public Domain)
 Casey, John: "The First Six Books of the Elements of Euclid"
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"