When mentally rotating a puzzle piece to find its fit, which transformation is most important to check systematically?

Fitting-parts questions require checking both rotation (0°, 90°, 180°, 270°) and reflection (mirror flip). Many test-takers forget the flip — if a piece doesn't fit at any rotation, try it mirrored. Not all shapes are symmetric, so mirroring produces a genuinely different shape that may match the target.