Linear polarisation Stokes parameters of four Sgr A* NIR flares observed by GRAVITY during 2018. The prime notation denotes the quantities as recorded by the instrument (including the effects of field rotation and systematics). Top left: Stokes Q′ on May 27. Top right: Stokes Q′ on June 27. Bottom left: Stokes Q′ on July 22. Bottom right: Stokes Q′ and U′ on July 28. All of the flares show ≳10 − 40% linear polarisation. A common, continuous evolution is seen on all nights. In three cases, Q′ shows a change in sign, consistent with rotation of the polarisation angle. The implied period of the polarisation evolution matches what is seen in astrometry.