Tasapinnalise joone kõverus on positiivne, kui vektor kooduva ringjoone keskpunktist vaadeldavasse punkti on kollineaarne ja normaalvektoriga samas suunas, ning negatiivne, kui vektor koolduva ringi keskpunktist vaadeldavasse punkti on kollineaarne ja normaalvektoriga vastassuunaline.
The curvature of a planar line is positive when the vector from centre of the osculating circle to the considered point is collinear and the same direction to the normal vector, and is negative when the vector from the centre of the osculating circle to the considered point is collinear and opposite to the normal vector.
