A credit system is used to enable members to trade books for credits and credits for books. Credits may either be purchased, or they may be earned by mailing books which have been requested. Consequently, a member need not seek another member who desires a parity trade; credits are the medium of exchange
. Both paperback
books may be traded, as well as audiobooks
. Within the PBS system the value of any bound book is one credit, and the value of an audio book is two credits.