Some of the commercial yarns will need to be altered for optimum results. Finished knotting cords for knitted or crocheted garments should be softer and more flexible. The knotting cord should be large enough not to disappear in the texture pattern of the garment. The yarn needs to be balanced when finished. A fuzzy or textured yarn will not show the knots off well but in some cases is suitable for a particular project.
Necessary strength is dependent on the intended use.
A cord may be weak if there will be no tension will be on the final knots. Generally speaking, elasticity is not desirable.