Flat spline
A spline, or the more modern term flexible curve, consists of a long strip fixed in position at a number of points that relaxes to form and hold a smooth curve passing through those points for the purpose of transferring that curve to another material.
Before computers were used for creating engineering designs, drafting tools were employed by designers drawing by hand. To draw curves, especially for shipbuilding, draftsmen often used long, thin, flexible strips of wood, plastic, or metal called splines (or laths, not to be confused with lathes). The splines were held in place with lead weights (called ducks because of their duck-like shape). The elasticity of the spline material combined with the constraint of the control points, or knots, would cause the strip to take the shape that minimized the energy required for bending it between the fixed points, this being the smoothest possible shape.
Splines are more recently referred to as flexible curves and perform much of the original function. The main difference between splines and flexible curves is that the control points of flexible curves are entirely internal in their housing. This has one advantage over splines: whereas the draftsman had to first set up the spline and control points before moving the object to be marked to the spline in that order only, with flexible curves the draftsman can also set up the flexible curve before carefully moving the flexible curve to the object to be marked.