Synchrotron spectral index as a function of rotation angle, computed using a T–T plot between the COSMOGLOBE DR1 K-band and the COSMOGLOBE DR1 30 GHz pixels (black) compared to the spectral index using the WMAP9 K-band and Planck PR3 30 GHz (red) pixels for all regions. The horizontal (solid and dotted) lines indicates the corresponding inverse variance weighted values of the spectral index, averaged over rotation angle, and in the COSMOGLOBE case also samples.