The various factors constraining the choice of synthetic polymer and natural gut strings for musical instruments are discussed, including a newly-formulated constraint based on the internal damping of the string material. It is shown that all these constraints can be summarised graphically in a design chart, and calibrated versions of the chart are presented for monofilament strings of nylon, fluorocarbon and natural gut. Based on these charts, detailed case studies are presented for the stringing of a lute and a harp. An explanation is suggested for why harpists continue to favour gut over nylon.