Illustration of the projected stellar disk for a star seen equator-on to its rotation. The equator is indicated. The “bisector line” defines zero longitude (φ = 0°) on the star. The red line represents the constant co-latitude of rotation for the magnetic axis at the point of interception with the stellar surface. The blue dotted circle indicates the locus of points which would have a constant polarization p if the magnetic pole were located on that curve, with the two blue dots as examples.