Adobe Photoshop CC 2014 v.14.xx - Manual (Page 28)

The selected edge is straightened. Also, the straightening of the edge is preserved during further perspective manipulation.
Shift-click the edge again if you don't want to preserve its straightening.
In the Warp mode, you can click the following icons for automatic perspective adjustment:
Automatically level near horizontal lines
Level horizontally
Automatically straighten near vertical lines
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