This 4GB Blank Cartridge can hold about 400 hours of talking book audio and can be used to store & play books that are downloaded from the National Library Service BARD website. | Now available in 4GB, 8GB and 16GB to give you more audio storage for less money with a new raised print feature to allow the visually impaired to quickly determine the cartridge capacity. | This cartridge works with the American Printing House for the Blind’s Book Port DT, and APH's Joy Player. (Note: APH's Joy Player is not enabled to play NLS Talking Books but is compatible with MP3 and Daisy file formats | This cartridge is primarily used for blind, visually impaired, or reading disabled people that are registered with the National Library Service program through each state’s affiliated library. | Be sure to check our full line all of products for the NLS Talking Book Player including our new line of Perkins Library brand cartridges. Note: All of our items ship using “Free Matter for the Blind” as postage. Items shipped using “Free Matter” as the postage method cannot be tracked. Typically our products arrive to our customer in 3-5 business days.