The tennis warehouse site states: Polyester - a very durable string designed for string breakers-not much power or feel. Polyester strings became very popular with ATP players, because it provides added durability, doesn't move and "deadens" the string bed. While this isn't a desirable feature for most recreational players, it is for many of today's ATP and (some) WTA players. They're bigger, stronger, swing faster and use more powerful rackets than players from the past. Often used in hybrids, combining polyester mains with softer synthetic or natural gut mains. This offers the durability benefits of polyester, while reducing the stiff, dead feel. Also easier to string than 100% polyester. Not recommended for beginning players or players with arm injuries. I have seen post after post on this forum where polyester strings break because of shearing. My question is why would anyone want to use a string that is supposed to last longer that breaks sometimes before you ever get a chance to hit with it? "Advertising is the art of convincing people to spend money they don't have for something they don't need." -Will Rogers Irvin PS - I am using Dunlop Explosive Polyester 17 Ms / Wilson K Gut 16 Xs in my Prince Hybrid Tour racket. Right now I like it but let it fail me and you will see a 600' reel of Dunlop Explosive Polyester 17 on ****. I have done three hybrid string jobs with it hence 600' reel.