Synchrotron spectral index as a function of region number, computed using T–T plots between the WMAP9 K-band and Planck PR3 30 GHz (red), WMAP9 K-band and Planck PR4 30 GHz (blue), WMAP9 K-band and BEYONDPLANCK 30 GHz (purple), and COSMOGLOBEK-band and COSMOGLOBE 30 GHz (black) maps. The spectral index is inverse variance weighted over rotation angles, and samples. The horizontal line in the high latitude regions corresponds to the estimated spectral index values from the Planck PR3 likelihood analysis (Planck Collaboration V 2020).