The definition of "postage stamp" can also be problematic for catalogers. For instance, some countries have issued adhesive labels purporting to be postage stamps, but which had the "cancel" printed directly on the stamp and shipped to dealers, without ever being sold to the public for use on letters. The treatment of these has long been a vexing issue, and catalogs vary greatly on whether to list the stamps. A related issue is a small number of extremely rare stamps that may or may not be old forgeries; the assignment or removal of a number is a key step in the consensus as to their authenticity.