Example of a candidate sympathetic event observed by SDO/AIA at 131 Å on October 23–24, 2013, with panels a to c showing the chronological evolution of the flaring regions. In panel a, the white and red lines represent a PFSS extrapolation made using the pfsspy code from Stansby et al. (2020), with the source surface radius at r_SS = 2.5 R_⊙. In panel c, the white arrow indicates the location of a loop brightening connecting the two active regions (NOAA 11875 and NOAA 11877). An animated version of this figure is available online.