The rule tool allows to correct the angle of a pic by adjusting a line on the horizon or on a vertical wall in the pic.
Clic on the pic for a larger size
Start the Trimming/Angle Adjustement Tool of DPP.
Clic on the “Show Rule” checkbox to show/hide the rule.
Clic on the white squares to adjust the rule on the horizon.
Clic on the green squares to apply the rotation.
Clic on the red squares to hide the rule.
A faster method to use the rule is :