A new Preview Pane feature is now available in Gmail lab. This allows Gmail users to preview their email messages and making reading an email faster.Preview Pane feature can be enabled using the Mail Settings. Click the Options icon on the top right corner of Gmail page and select Mail Settings from the displayed list.
In the Mail Settings page, navigate to Labs section then scroll down to Preview Pane feature.
To enable Preview Pane, select Enable radio option and click the Save Changes button.
Gmail will now have a Toggle Split Pane mode with Vertical, Horizontal and No Split option as show above. Users can select the desired split mode and preview the email messages. Shown below is an example of horizontal split mode preview pane.