Two-Panel Selector

MacOS System Preferences background image selector


Put two side-by-side panels on the interface. In the first one, show a list of items that the user can select at will; in the second one, show the content of the selected item.

Use when

You have a list of items to show. Each item has interesting content associated with it, such as the text of an email message, a long article, a full-sized image, contained items (if the list is a set of categories or folders), or details about a file’s size or date.

You want the user to see the overall structure of the list and keep that list in view all the time, but you also want him to be able to browse through the items easily and quickly. People won’t need to see the details or content of more than one item at a time.

Physically, the display you’re working with is large enough to show two separate panels at once. Very small cell phone displays cannot cope with this pattern, but many larger mobile devices can.

Compare to: One-Window Drilldown, List Inlay


This is a learned convention, but it’s an extremely common and powerful one. People quickly learn that they’re supposed to select an item in one panel to see its contents in the other. They might learn it from their email clients, or from Windows Explorer, or from websites; whatever the case, they apply the concept to other applications that look similar.

When both panels are visible side by side, users can quickly shift their attention back and forth, looking now at the overall structure of the list (“How many more unread email messages do I have?”), and now at an object’s details (“What does this email say?”). This tight integration has several advantages over other physical structures, such as two separate windows or One-Window Drilldown:

  • It reduces physical effort. The user’s eyes don’t have to travel a long distance between the panels, and he can change the selection with a single mouse click or key press rather than first navigating between windows or pages (which can take an extra mouse click).
  • It reduces visual cognitive load. When a window pops to the top, or when a page’s contents are completely changed (as happens with One-Window Drilldown), the user suddenly has to pay more attention to what he’s now looking at; when the window stays mostly stable, as in a Two-Panel Selector, the user can focus on the smaller area that did change. There is no major “context switch” on the page.
  • It reduces the user’s memory burden. Think about the email example again: when the user is looking at just the text of an email message, there’s nothing on-screen to remind him of where that message is in the context of his inbox. If he wants to know, he has to remember, or navigate back to the list. But if the list is already on-screen, he merely has to look, not remember. The list thus serves as a “You are here” signpost (see Chapter 3 for an explanation of signposts).
  • It’s faster than loading a new page for each item, as can happen with One-Window Drilldown.


Place the selectable list on the top or left panel, and the details panel below it or to its right. This takes advantage of the visual flow that most users who read left-to-right languages will expect (so try reversing it for right-to-left language readers).

When the user selects an item, immediately show its contents or details in the second panel. Selection should be done with a single click. But while you’re at it, give the user a way to change his selection from the keyboard, particularly with the arrow keys—this helps reduce both the physical effort and the time required for browsing, and contributes to keyboard-only usability (see Keyboard Only in Chapter 1).

Make the selected item visually obvious. Most GUI toolkits have a particular way of show- ing selection (e.g., reversing the foreground and background of the selected list item). If that doesn’t look good, or if you’re not using a GUI toolkit with this feature, try to make the selected item a different color and brightness than the unselected ones—that helps it stand out.

What should the selectable list look like? It depends—on the inherent structure of the content, or perhaps on the task to be performed. For instance, most filesystem viewers show the directory hierarchy, since that’s how filesystems are structured. Animation and video editing software use interactive timelines. A GUI builder may simply use the layout canvas itself; selected objects on it then show their properties in a property editor next to the canvas.

A Two-Panel Selector has identical semantics to tabs: one area for the selectors, and one area next to it for the content of the selected thing. Likewise, a List Inlay is like an Accordion (Chapter 4), and One-Window Drilldown is like a Menu Page (Chapter 3).

When the select-and-show concept is extended through multiple panels to facilitate navi- gation through a hierarchical information architecture, you get the Cascading Lists pattern.


Many email clients use this pattern to show a list of email messages next to the currently selected message. Such listings benefit from being nearly as wide as the whole window, so it makes sense to put the listing on top of the second panel, not to its left. (Also, this example shows the use of a third selector panel on the left that lets the user choose which mailbox to work in.)

Mac Mail

Like many other Picture Managers, Picasa lists the various image folders and categories in its Two-Panel Selector. The result is a second list, of images. When the user selects an image, the whole window is replaced; see One-Window Drilldown.


In other libraries

Overview by Detail at
Two-Panel Selector at Quince