Ratio of predicted 1σ uncertainties (see end of Sect. 5.2) showing how constraints are tightened after adding CMB lensing (blue) or all CMB probes (orange) when compared to the Euclid-only constraints (black outer rim), assuming a pessimistic Euclid scenario and SO-like CMB data, for four selected cosmological models (from top to bottom, left to right: flat ΛCDM; flat w₀w_aCDM; non-flat ΛCDM; and flat w₀w_aγCDM).