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Or is not necessary and why? Where is the definition?

Carlitos_30
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  • According to Wikipedia, two lines are perpendicular if they meet or intersect at a right angle – J. W. Tanner Dec 26 '19 at 16:01
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    My school geometry defines two lines $d,e$ in space to be perpendicular, iff after a parallel transport of the one line $e$ so that the "moved line" $e'$ intersects the other one, we have in plane $d\perp e'$. This terminology is useful when dealing with the theorem of three perpendiculars, or when claiming "if a line $d$ is perpendicular on two lines $e,f$ generating a plane $\pi$, then $d\perp \pi$. – dan_fulea Dec 26 '19 at 16:09
  • I'm not sure what you mean by "perpendicular between them". If two lines are skew, then there is a line perpendicular to both of them. – Gerry Myerson Dec 26 '19 at 17:31
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    Maybe this may shed light on your question. – Andrew Chin Dec 26 '19 at 17:53
  • Any thoughts on the comments, Carlitos? – Gerry Myerson Dec 28 '19 at 03:58
  • Thank you very much. Andrew Chin link clarified my doubt. – Carlitos_30 Dec 30 '19 at 04:57

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