Google enabled a new lab feature -email preview pane- for Gmail to compete against their arch rivals like Microsoft’s Hotmail, Yahoo! Mail and other Webmail programs.The Preview Pane lets you preview your Emails without opening them, and allows you to quickly browse through all your emails quickly. With this three-window panel layout , users can read or reply to one email while also being able to see a preview of any other email that land in their inbox.
You can go into the Gmail Labs tab under Gmail and enable the option.With this you can alter the panes between vertical and horizontal orientations.You can enable this using the dropdown button .It is in the upper right-hand corner, which allows the user to switch between the traditional list view and the new preview pane mode.
Gmail also allows you to adjust how many seconds you can spend looking at a message's preview before that message gets marked as read; the default is 3 seconds.