Talking books require the use of a specialized playback device. In 2009, digital format books were introduced on easy-to-handle cartridges. Two types of digital players are available: a standard model and an advanced model with navigation and bookmark features. Current readers may use digital players to access the full range of the NLS collection. NLS formats render the books unusable by the general public, a requirement under the U.S. copyright law to protect intellectual property while allowing NLS patrons free use of the material.