XY Orientation Calibration piece

Every printer i've built so far has needed one of these at some point,

Most of the time when you're printing Symmetrical objects (rotational or otherwise) you won't notice if your X or Y axis are inverted.
The words should be readable from the front of your machine - if they're mirrored in either direction then the chances are that you need to invert that axis direction (and adjust or flip direction on your endstops accordingly)

Also useful for checking / simple bed levelling