Skewed prints (squares come out as parallelograms)

Corners aren't 90°, so square parts lean slightly and the two diagonals measure different lengths.

Also described as skew, not square, parallelogram, leaning, diagonals different, out of square, corners not 90

Try these in order

Most likely and quickest to check first. Change one thing at a time so you know what fixed it.

Frame or gantry not square
If the X and Y axes aren't exactly at right angles, every print leans the same way.
- Print a large square, measure both diagonals, and check the frame with a set square.
- Loosen and square the gantry, then retighten it evenly.
Firmware or slicer correction not set
- Use skew correction in the firmware (M852 in Marlin, SET_SKEW in Klipper) using your diagonal measurements.
Belts at different tensions
On CoreXY printers, uneven belts skew the gantry.
- Set both belts to the same tension.