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'''Perpendicular lines''' are lines which intersect to form four [[right angles|<math>90^o</math> angles]].
'''Perpendicular lines''' are lines which intersect to form four [[right angles|90<sup>o</sup> angles]].<ref>''Merriam-Webster's Third International Dictionary''.</ref> In Euclid's ''Elements'', they were defined as lines which intersect to form one right angle,<ref>Euclid. ''The Elements''. Book I.</ref> and they were proven to form four via the [[Perpendicular Lines Theorem]].
[[category:mathematics]]
==References==
{{Reflist}}
[[Category:Plane Geometry]]