Field-line helicity of a braided magnetic field, viewed in different cross-sections. The topology of contours and critical points is preserved, but the pattern appears simpler in the middle cross-section, and hence the 42 critical points are easier to find there. The topology of this pattern is important because it is also preserved (to leading order) in a resistive relaxation of the underlying magnetic field. Understanding what final states can be reached under such constraints is the theme of my Leverhulme Trust project...