The signs of a pair tell its corner: (+, +) is the top right, quadrant 1. (−, +) is the top left, quadrant 2. (−, −) is the bottom left, quadrant 3. (+, −) is the bottom right, quadrant 4.
A mirror changes the signs: across the x-axis flips the sign of y, and across the y-axis flips the sign of x.
Across both axes flips both.