Google released Gmail for tablet devices on April 2, 2010. If you used the Gmail on tablet devices, probably you might be familiar with the Gmail's layout - similiar, to Gmail for tablet, you can now preview messages in your inbox using a new feature in Gmail Labs called "Preview Pane."
Here's how the Preview Pane looks: (click to enlarge)
To use this feature, first you have to enable Preview Pane by going to the Labs tab in Gmail Settings there you will see a toggle button in the top right corner of your message list, which lets you switch between preview and list views. See the screenshot below:
For those of you who have more vertical space you can also move the preview pane below your message list. You can enable this using the dropdown arrow next to the toggle button. You have three choices to choose from: 'No Split, Vertical Split, or Horizontal Split'. Take you own pick. See the example picture below:
By default there is a 3-second delay in marking a conversation as read after previewing it. If that doesn't feel natural to you, you can change the timing in the General tab of settings:
If you'd like to remove the new layout, simply return to Gmail Settings and disable the lab.
[Source: Gmail blog]