Upper panel: mean longitudinal magnetic field of HD 166473 against rotation phase. The different symbols identify the sources of the ⟨B_z⟩ values, as follows: open squares (brown) or filled triangle (violet): Mathys & Hubrig (1997, CASPEC); filled circles (dark blue): Mathys (2017, CASPEC); four pointed open stars (light green): Bagnulo et al. (2015, FORS-1); filled pentagon (yellow): this paper (HARPSpol); filled squares (salmon): this paper (ESPaDOnS). The long-dashed line (red) is the best fit of the CASPEC measurements of Mathys (2017) and of the HARPSpol and ESPaDOnS data by a cosine wave (see Eq. (5)). The short-dashed line (green) corresponds to the superposition of low-order multipoles discussed in Sect. 4. Lower panel: differences O–C between the individual ⟨B_z⟩ measurements and the best-fit curve, against rotation phase. The dotted lines (blue) correspond to ±1 rms deviation of the observational data about the fit (red dashed line). The symbols are the same as in the upper panel. The CASPEC measurements of Mathys & Hubrig (1997) and the FORS-1 measurements are not shown in this panel as they were not included in the fit (see text).