Editing Subtangent (section)

{{Short description|Mathematical concept}}
[[Image:SubtangentDiagram.svg|thumb|300px|right|Subtangent and related concepts for a curve ('''black''') at a given point ''P''. The tangent and normal lines are shown in <span style="color:green;">green</span> and <span style="color:blue;">blue</span> respectively. The distances shown are the <span style="color:#800000;">'''ordinate'''</span> (''AP''), <span style="color:#008000;">'''tangent'''</span> (''TP''), <span style="color:#808000;">'''subtangent'''</span> (''TA''), <span style="color:#000080;">'''normal'''</span> (''PN''), and <span style="color:#800080;">'''subnormal'''</span> (''AN''). The angle φ is the angle of inclination of the tangent line or the tangential angle.]]
In [[geometry]], the '''subtangent''' and related terms are certain line segments defined using the line [[tangent]] to a curve at a given point and the [[Cartesian coordinate system|coordinate axes]]. The terms are somewhat archaic today but were in common use until the early part of the 20th century.