Dacron (strength per strand = 22.5 kg (50 lb), stretch = 2.6%), a polyester material. Because of its durability and stretch, Dacron is commonly used on beginners' equipment, wooden bows, and older bows. The relatively high stretch causes less shock to the bow, which is an important consideration for wooden-handled recurves. Dacron strings are easy to maintain and can last several years.
Liquid crystal polymers such as Kevlar and Vectran (strength per strand = 31.8 kg (70 lb), stretch = 0.8%) are polymer materials with a higher density and smaller diameter than Dacron, which results in a faster arrow speed (approximately 2 metres per second (6.6 ft/s) faster). There are two problems with this material. First, its limited stretch causes increased stress in the bow limbs. Secondly, a Kevlar bowstring may only last 1000 shots before breaking as it tends to fatigue due to bending at the nocking point. Failure tends to be sudden rather than gradual.
Ultra-high-molecular-weight polyethylenes, such as Spectra and Dyneema (strength per strand = 45.5 kg (100 lb), stretch = 1.0%), have been used since the 1990s. They are lighter, therefore faster, than Kevlar—and have a much longer life.
Modern strings are often made from composite fibres—such as a mixture of Vectran and Dyneema—to gain the advantages of both.