I don't know, but the best I can deduce, the one in one out rule says that if you buy a pipe, you have to get rid of a pipe, to keep from accumulating an endless number. Others have called this culling the herd, when you round up a few pipes you're not smoking and sell them or trade them in for retailer credit on an upgraded pipe, or just a pipe you like. Whatever works for you. Some just grin and bear it, and own a few hundred pipes. Others are totally happy with six, or fifteen, or thirty-five, or eighty, and leave it at that. Stay away from pipe shops, pipe clubs, pipe shows, pipe web sites, pipe catalogs, etc.