Most yarn bands will include a line about the recommended needle and sometimes recommended hook size for the yarn. These recommendations go hand in hand with the gauge description that's often also on the band. That is, if you use that needle or hook size--generally speaking--the average knitter or crocheter should be able to get a particular number of stitches per inch. This can be particularly helpful information when trying to determine yarn substitutions; however, there is a caveat. This information is not supposed to lock you into a particular needle or hook size. Let me repeat: the recommended needle/hook size is NOT supposed to lock you into a particular needle or hook size. Too often I've seen people discard a potential yarn simply because the recommended needle size doesn't match the pattern without a second thought and then become frustrated when they can't find a yarn--even the pattern-recommended yarn--where the yarn band's recommended needle/hook size doesn't match the needle or hook used in the pattern. FYI: Most patterns out there do not use the recommended needle or hook size for a yarn. This is because they often attempt to achieve a different kind of resulting fabric than the "average" fabric that you would make with correct gauge and the recommended needle or hook. And, that's okay! Elizabeth Zimmermann used to say that needle and hook size is largely arbitrary because each person is different! You may need to go up or down a size or two (or four) to get the same gauge for the project as the designer. That is okay! But, it's also reason to make sure that you don't lock yourself into the recommended size on the yarn band.