My chair legs seemed between sizes so I bought the smaller ones for a tight fit.It's a very tight fit These glides were quite hard to get on, but that seems a good thing because nothing wants to stay on these chairs. Their legs are round, almost 1 1/8", and have an obnoxious somewhat rounded taper. The legs are also angled and not completely flat to the floor except to the extent they have worn flat at edges on the previous floor. Yes, they should be sanded off to flat, but they are old, and so far, I have more pressing jobs to do.Because of the uneven leg bottoms, and because the felts on these small-sized glides are smaller than the leg bottoms, I have opted to leave my old nail-on plastic glides on the legs and put these new silicon ones right over them. The nail-on glides are on solidly and are flat to the floor.After about two months, there is no slippage at all.My concern is that the fairly thin felt is still a little smaller than the contact area of the chair leg/plastic glides. It looks like it will compress, silicon could touch down and they will drag crud and wear through.BUT maybe not. If/when that happens, I might order these again in the larger size. They were so tight going on there's a chance the larger ones will still be tight enough.I have one test chair without the plastic glides but I only just put the new glides on it. I'll update if there is anything enlightening about that experiment.I am terribly impressed that something is finally staying on these chairs.