Compare an edit with the original

Check an edit against the original photo as you work: flip between them, hold a key to peek, or see before and after side by side.

B, or the original button in the toolbar, shows the original until you press it again.
Hold Shift to peek at the original for as long as the key is down — handy with the other hand still on a slider.
Y, or the side-by-side button, shows the original on the left and the edit, live, on the right. It works on one photo; from a sheet it takes the highlighted one. Press Y again for one photo.
Hold Space for a full-screen look at the highlighted photo with the edit applied, and Shift+Space for the same look at the original. Let go to come back.
I opens the Info tab with the file facts for the highlighted photo.

The toolbar buttons light up while they are on, so the toolbar always says whether you are looking at the original or the edit.