Off-line mail readers are generally considered to be those systems that did not originally offer such functionality, notably on bulletin board systems where toll charges and tying up telephone lines were a major concern. Users of large networks such as FidoNet regularly used offline mail readers, and it was also used for UseNet messages on the internet, which is also an online system. The two most common formats for FidoNet BBS's were Blue Wave and QWK. Less well-known examples include Silver Xpress's OPX, XRS, OMEM, SOUP and ZipMail.