Summary of observations of the SOL2021-05-07T18:43 M3.9 flare. a) Positions of XRT, STIX and the flare (not to scale) as seen from the ecliptic north pole. b) and c) Full-disk views of the Sun in the AIA 1600 Å channel as seen from the positions of STIX and XRT. The orange boxes highlight the flaring active region (NOAA AR 12822). d) Light curves of the flare X-ray emission as measured in the GOES/XRS 1–8 Å channel (black), STIX 22–50 keV (blue) and 6–10 keV (red) bands, and the XRT Be-thick filter (orange). The dashed vertical lines show the interval for which 3D reconstruction was performed. Due to the different light-travel times from the active region to the two spacecraft, the times in this figure have been transformed to light-departure time, i.e. the time at which the emission left the active region. e) and f) Images of the thermal flare loop-top source (red contours) taken by STIX (panel e, 6–10 keV) and XRT (panel f, Be-thick filter), overlaid on AIA 1600 Å observations of the flare ribbons. The non-thermal footpoints imaged by STIX (blue contours, 22–50 keV) are also shown in panel e.