My used boat came with 7/16" main and jib sheets. I needed to replace them both with longer lines and got the same diameter. I'm thinking of downsizing them (next smaller size would be 3/8") for various reasons: Main: Lots of friction. I have to push the boom out in anything but heavy air. The upside is that with that much friction I can often sail with the main uncleated. Seems like a saftey feature in a way, but I'm thinking it's becoming tedious to have to push the boom out. Jib: 1) I have an option to buy a Forespar 6-12 ADJ-DL whisker pole but the end fittings are too small for my large jib sheets. 2) I suspect that the Spinlock PowerCleats on the jib sheets will work better with a smaller diameter. The large diameter changes the angle of the internal cam cleat. I think this may have contributed to breaking one last year. 3) Friction could become a problem in very light air as the jib sheets get older and fuzzier. What size lines do you all use? ...RickM...