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Re-categoized, expanded definition
'''Perpendicular lines''' are lines which intersect to form four [[right angles|90<sup>o</sup> 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]]
[[Category:Plane Geometry]]