Hi Guys I would like to talk about a specific Zero Tolerance Knife.  I love zt. I have a 350 myself as an edc however I would like to talk about the zt 550 this knife really puzzles me! have you tried flipping it? The knife didn't have a flipper, which is fine, I get that, but ironically it didn't have an official thumb stud either! it had a blade stop that most people confused with a thumb stud. The difference is that an official thumb stud there's a space between the handle and the blade itself, on some knives, they make these bladestop/thumbstuds which are actually called blade stops whose function is to stop the blade from going all the way upward when flipping it.  That's fine, but on most knives they have a flipper too giving you an official way to open the knife, either that or the thumb stud is really a thumb stud and they put a bar across the top of the knife to prevent it from going up. What i'm saying is every knife has a definitive way to flip it.  It either has a defined thumb stud or a flipper, or a thumb stud/blade stop but there's a flipper too. On the zt550, there's no flipper and there's no thumb stud, just a blade stop that you think is a thumb stud and they leave you with only that, to flip open the knife. The result, you can either flip it or you can't.  Anyhow it took me 3 tries to flip that knife and it was hard! I mean real hard to flip. on the third try I flipped. which was great because my thumb started to kill me.  I was a sportco store and the guy in the knife section said he couldn't flip it either and never bothered to try. On the knife there is what what appears to be the resemblance of a flipper??? but actually it doesn't protrude enough to be any use to you. I guess zt550 (Which is now discontinued thank god!!) was made not to be used at all??? since it doesn't have a flipper or an official thumb stud?? I think this is an interesting discussion maybe you guys might have a different take on it.  either way some people can flip easily, other like me it takes a few tries and other guys like the poor fellow at the sport co store, can't flip it at all.  In any case i will include a pic of this knife and tell me guys what you think. The zt550 however, If you can flip it open does have a really smooth opening, for a knife that does not have I think, correct me if I'm wrong guys, a ball bearing system. maybe one of you maybe more knowledgeable on opening systems than I.  Anyhow this is an interesting discussion about a knife that really made me think about how knives are designed.  Like I said the zt550 is a puzzler. The designer is rick hinderer.  Cant wait for your comments!