Lightroom - Working With Before/After Views

Before/After Views give you a holistic perspective of how all the changes you have done in the Develop Module have changed your original image. To access them, select the Before/After icon illustrated here at the bottom of the Loupe View window. You can also use the shortcode Y. Each time you select the Before/After icon (or press Y) you toggle between horizontal and vertical image overviews; with the image either side by side or split. Left-clicking whilst in the Before/After view lets you zoom-in simultaneously on both images.

You can exit the Before/After view by selecting the Loupe View icon at the bottom left of the preview window, or using the short-code D.