Illustration of MRS coordinate systems. Left: detector coordinates contain the 2D spectra projected by the IFU on the MIRI detectors as curved slices. Middle: local MRS slicer coordinates are changing from band to band and represent the alignment of the slicer optic, with α being the along-slice and β the across-slice coordinate. Right: JWST coordinate system V2+V3 is defined by the JWST Optical Telescope Element (OTE) and is shared among all instruments. Note: this panel is not to scale with the actual dimensions since the MRS FoV is very small.