"Protagon"
Tennis racket with adjustable tension (1983)
Protagon-tennis rackets with the Bergelin-longstring-system are rackets that have all the strings joined in the grip so with a special tool the tension of the strings can be adjusted.
Sometime after the last stringing the tension is loosening up on all rackets. Also different conditions (court, balls and influences like wind) need different tensions to play perfectly. Therefor most professionals have at least 3 rackets with them on the tennis court. The stringing itself is also easier with the Protagon-system because no machine is needed.
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Source
Worldwide 80.000 rackets with this system were sold in total.