Three-pane interface

The three-paned main window of Mozilla Thunderbird.

A Three-pane interface is a category of graphical user interface in which the screen or window is divided into three panes displaying information. This information typically falls into a hierarchal relationship of master-detail with an embedded inspector window. Microsoft's Outlook Express email client popularized a mailboxes / mailbox contents / email text layout that became the norm until web-based user interfaces rose in popularity during the mid-2000s. Even today, many webmail scripts emulate this interface style.