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This category contains definitions related to Perpendiculars.
Related results can be found in Category:Perpendiculars.

In the words of Euclid:

When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right, and the straight line standing on the other is called a perpendicular to that on which it stands.

(The Elements: Book $\text{I}$: Definition $10$)


In the above diagram, the line $CD$ has been constructed so as to be a perpendicular to the line $AB$.