From the PDGA website:
803.09A (cont.) The out-of-bounds line itself is considered out-of-bounds.
This simple statement seems to baffle even experienced players, possibly because our OB lines can be all widths from razor thin edges to thick uneven lines and comprised of all kinds of materials from posts to curbs to fences to paint and to string.
Our club has generally played 15C/16's OB as the perimeter of the trees, and also marked it as such when marked for tournaments. I just really killed my own argument about 3 and 5's OB line however, where the peeler cores are VERY CLEARLY THE OB LINE on a daily basis, but when marked for tourney are generally strung on the outer edge. When there is string in place it functions as the OB line. When there is no string the peeler core is the line, not part of the fairway, thus reducing the fairway width by the 4 inches that comprise the line.
Remember when we played that if you were touching grass, you were in. Everyone argued about that, too.