Click on the previous/next step in the History panel, or press Ctrl+Z/Ctrl+Shift+Z. (Cmd on a Mac)
If your "before" image is the original image, you can press the backslash key.
Backlash doesnt work.
Not to be confused with the forward slash key?
Oh. I confused it. Thanks
The back slash key isn't working for me. I'm not pressing the forward slash. I just upgraded to CC so is there a different shortcut key?
In CC key shortcuts are the same-
In Grid mode [G] you can "Compare" two selected photos [C] (this could be Master & Virtual Copy)
In Grid mode [G] you can look at several selected photos together on screen in "Survey" mode [N]
In Develop module [D] you can-
See Before/After together in a split screen [Y] The Drop-down menu can change layout left/right, top/bottom, etc [Y/Y]
or Toggle the visible image between Before and After views with the back-slash key [ \ ]
*The "Before" view may be by default the image view at the time of Import, but you can change this to show "Before" as any step in the develop history. Right-click the step in history panel > "Copy History Step settings to 'Before'"
You may want to see Before/After as the image changes with steps in a Monochrome developing process, (and not see the original color version!)
If "back slash key isn't working" you may need to do above -Copy history step. (assuming you have done some developing steps, and have not cleared history!)