 The top professionals of the 1970s and earlier, despite having access to stiffer materials such as nylon, nearly always chose to use the very flexible natural gut instead.String bed stiffness can be increased by using stiffer materials, such as kevlar and polyester, by increasing the density of the string pattern, and by stringing with a higher tension.Racket makers and players have experimented with very dense string patterns and very "open" patterns, beginning with the Snauwaert Hi Ten, which had a pattern with as few as 12 mains and 13 crosses.