Editing Line (geometry) (section)

=== Relationship with other figures===
[[File:Tangent to a curve.svg|alt=see caption|thumb|Tangent to a curve. The red line is tangential to the curve at the point marked by a red dot.]]
In Euclidean geometry, all lines are [[congruence (geometry)|congruent]], meaning that every line can be obtained by moving a specific line. However, lines may play special roles with respect to other [[geometric object]]s and can be classified according to that relationship. 

For instance, with respect to a [[Conic section|conic]] (a [[circle]], [[ellipse]], [[parabola]], or [[hyperbola]]), lines can be:
* [[tangent line]]s, which touch the conic at a single point;
* [[secant line]]s, which intersect the conic at two points and pass through its interior;<ref name="cag">{{citation |last1=Protter |first1=Murray H. |title=Calculus with Analytic Geometry |url=https://books.google.com/books?id=jTmuOwwGDwoC&pg=PA62 |page=62 |year=1988 |publisher=Jones & Bartlett Learning |isbn=9780867200935 |last2=Protter |first2=Philip E. |author1-link=Murray H. Protter}}</ref>
* exterior lines, which do not meet the conic at any point of the Euclidean plane; or
* a [[Directrix of a conic section|directrix]], whose distance from a point helps to establish whether the point is on the conic.
* a [[coordinate line]], a linear coordinate dimension

In the context of determining [[parallel (geometry)|parallelism]] in Euclidean geometry, a [[transversal (geometry)|transversal]] is a line that intersects two other lines that may or not be parallel to each other.

For more general [[algebraic curve]]s, lines could also be:
* ''i''-secant lines, meeting the curve in ''i'' points counted without multiplicity, or
* [[asymptote]]s, which a curve approaches arbitrarily closely without touching it.<ref>{{citation |last=Nunemacher |first=Jeffrey |title=Asymptotes, Cubic Curves, and the Projective Plane |journal=Mathematics Magazine |volume=72 |issue=3 |pages=183–192 |year=1999 |citeseerx=10.1.1.502.72 |doi=10.2307/2690881 |jstor=2690881}}</ref>
With respect to [[Euclidean triangle|triangles]] we have:
* the [[Euler line]],
* the [[Simson line]]s, and
* [[central line (geometry)|central lines]].

For a [[convex polygon|convex]] [[quadrilateral]] with at most two parallel sides, the [[Newton line]] is the line that connects the midpoints of the two [[diagonal]]s.<ref name="Alsina">{{cite book |first1=Claudi |last1=Alsina |first2=Roger B. |last2=Nelsen |title=Charming Proofs: A Journey Into Elegant Mathematics |publisher=MAA |date=2010 |isbn=9780883853481 |pages=108–109}} ({{Google books|mIT5-BN_L0oC|online copy|page=108}})</ref>

For a [[hexagon]] with vertices lying on a conic we have the [[Pascal line]] and, in the special case where the conic is a pair of lines, we have the [[Pappus's hexagon theorem|Pappus line]].

[[Parallel (geometry)|Parallel lines]] are lines in the same plane that never cross. [[Line-line intersection|Intersecting lines]] share a single point in common. Coincidental lines coincide with each other—every point that is on either one of them is also on the other.

[[Perpendicular lines]] are lines that intersect at [[right angle]]s.<ref>{{citation |last1=Kay |first1=David C. |title=College Geometry |page=114 |year=1969 |location=New York |publisher=[[Holt, Rinehart and Winston]] |isbn=978-0030731006 |lccn=69-12075 |oclc=47870}}</ref>

In [[three-dimensional space]], [[skew lines]] are lines that are not in the same plane and thus do not intersect each other.