Using splines

[kjkrum]

When drawing splines, the control points (knots) fall outside the area enclosed by the curve, but I don't understand their exact relationship to the curve. Rather than randomly moving the knots around until my curve looks right, I want to understand how to place the knots deterministically so that my curve has the dimensions I want. I looked up spline tutorials, but quickly got into math I don't understand. Also, there are different types of splines. Anyone know what type QCad uses? Then at least I'd know which tutorials I should be looking at.

For example, say I want to draw an acoustic guitar body. I know the length, the widths at the waist and bows, and the distances along the center line at which lines drawn across the waist and bows would intersect it. I want to know how to place my spline knots to achieve these predetermined dimensions.