Several authors discuss accessibility as a similar concept to usability. Accessibility is often defined as barrier-free design. Usability is focused on the ease of use of a product for users. In general, accessibility is focused on older and disabled users while usability is focused on the typical user. These topics are usually discussed separately. However authors are increasingly interested in the intersection of accessibility and usability. Some, such as Burgstahler, et al., 2004 and Henry, Grossnickle, 2004, suggest using the principles of usability or user-centered design to achieve accessibility. Regardless of their specific stance, all these articles note that accessibility and usability are different but closely related concepts.