Polarised Galactic foregrounds remain the dominant systematic in the search for primordial CMB B modes. Beyond standard component separation, the morphology and frequency dependence of the polarisation P, decomposed into its additive E and B subcomponents, provide new, physically interpretable leverage on foreground contamination.

In this talk, I demonstrate how working directly with the PE and PB fields offers a powerful and intuitive framework for foreground characterisation (with respect to more standard Q, U, E or B fields). Map-level filtering of E- and B-like patterns cleanly disentangles Galactic emission from B-mode contaminants relevant to the tensor-to-scalar ratio. After illustrating these advantages with a toy model, I apply the method to data, including Planck, WMAP and C-BASS. This approach naturally reveals coherent astrophysical features such as polarised synchrotron loops and delivers high–signal-to-noise spectral parameter maps, enabling direct estimates of synchrotron-induced bias on r and strengthening the robustness of cosmological B-mode searches.