Chapter 6: User Interfaces for Mobile Media 239 Figure 6-21. A list can have a straight, curved or bent axis. the user wishes to change the selection, they open the list by clicking the arrow button attached next to the list item. After locating the desired item and selecting it, the list is closed and the chosen item presented. Not only lists can be dropped down: in Figure 6-19, the calendar’s monthly view can also be dropped down. There are also multi-selection list boxes that allow selection of more than one item. The selection is usually done by toggling on and off a checkbox attached to each list item. Alternatively, a mode switch, such as a menu command or a hardware key, can be used. Selected items are then identifi ed with a mark, often a “✔” character, after their label. A list is a one-dimensional structure; in other words, it has one axis. The axis does not necessarily have to be a straight line, as it can be curved or bent (Figure 6-21). Still, the navigation is preserved in one dimension. Often, only a small fraction of the content is visible at any one time on the small screen. As a result, the user may not know what choices are available without scrolling the view, which is a tedious task if the list contains a large number of items. Obviously there are techniques for aiding interaction with the lists. For example, the user may sort or fi lter the list based on some keyword or other criteria. If another visual axis is added, the presentation is called a grid, or sometimes a matrix or a table. The graphical content, such as images, is often shown in the form of a grid. In addition to graphics, a grid item may also contain one or more rows of labels. Since the visual structure and navigation space are two-dimensional, the user is able to see more content and access it with fewer navigation steps as when compared to a list (Figure 6-22). If the content objects form a hierarchy, it is typically presented with a tree view (Figure 6-23). A hierarchy related to music could consist of, for example, artists, their albums, and tracks in each album.