Spline — Single knots at 1/3 and 2/3 establish a spline of three cubic polynomials meeting with C2 continuity. Triple knots at both ends of the interval ensure that the curve interpolates the end points
Photo Credit: By Garry R. Osgood - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4314548