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When a straight line standing on another straight line makes the adjacent angles equal to one another, each of the angles is called a right angle; and the straight line which stands on the other is called a perpendicular to it.
Elements of geometry, based on Euclid, book i - Page 23
by Edward Atkins - 1877

## Geometry: Our Cultural Heritage

Audun Holme - Mathematics - 2002 - 408 pages
...the angle are straight, the angle is caller rectilinear. 9. When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the angles is right, and the straight line standing on the other is called the perpendicular to the one on which...

## Classics in the History of Greek Mathematics

Jean Christianidis - Mathematics - 2004 - 502 pages
...perpendicular to AT and Definition I.10, which states that “When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the right angles is right, and the straight line standing on the other is called a perpendicular to that...

## Visualization, Explanation and Reasoning Styles in Mathematics

Paolo Mancosu, Klaus Frovin Jørgensen, Stig Andur Pedersen - Mathematics - 2005 - 336 pages
...of (rectilinear) angles (trans. Heath 1926, I, 181): 1 0. 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 [...]. 1 1 . An obtuse angle is an angle greater than a right angle. 12. An acute...