Google has just added a new flexibility in its Gmail product by providing Preview Pane feature. If you have ever used Outlook or Mail App on Mac machines, you will be well aware of this feature. In fact it lets you to preview the message in inbox while eliminating the need of going back and forward to read a new email.
By default this feature is disabled, but you can enable it under Lab tab which can be accessed by clicking on Settings button available in the top right corner. Alternatively click on this link to directly lank on Lab Tab provided you are already logged into your Gmail account.
After you have enabled Preview Pane feature you can toggle between Preview and List view by using a new toggle button which will appear in the top right corner of messages list.
In case you have enough vertical space, you can also place the Preview Pane under your messages list. To do this use the down-arrow available next to toggle button.