The reference by SeanM and others to "arrangements" is possibly more weighty than it appears. Simple word and/or note alterations can make your "version" a "unique arrangement." A fine example is to look at Hal Leonard Corp.'s "Celtic Fake Book." All, or nearly all, the songs in it are labeled "traditional," and each one is independently marked as "Copyright 2000, Hal Leonard Corp." If you're attentive to subtle nuances, you'll pick up on subtle variations between their versions and others.