Constraint

Though the constraint principle is used throughout the Itunes design, the most obvious application is in the navigation of the Itunes library. When a user is viewing his or her music collection in “list view”, a scroll bar appears along the right side of application as shown in the screenshot below.

Using the scroll bar, the user can view the contents of his or her library by dragging the scroll bar up and down the right side of the application. The scroll bar stops when the user has reached the beginning or the end of the library.

The stop of the scroll bar is a physical constraint. It limits the range of actions a user can take by using a vertical, linear path to represent the size of the user’s library.