YDS stands for "Yosemite Decimal System," and is a classification for rock climbing. 5.5 YDS corresponds to 4+ SCA. I think it's either 4 or 4+. Something else to relate to: I think the final ridge up Gannett is no harder than the summit knob of Granite Peak, MT. We took rope for both Gannett and Granite, but never took it out for either one. The north ridge of Gannett is, if anything, a little easier than MT.