Coronal holes, like the one at the center of this September 25, 2019 Fe XII 193 A image, are regions where the magnetic fields are open to interplanetary space. They are the source of high speed solar wind streams that sweep past Earth, inducing geomagnetic activity and auroras. I will begin with an introduction to the basic properties of the Sun’s magnetic fields and then apply this understanding to the problem of how coronal holes evolve on the Sun. A central question is why coronal holes can sometimes defy the shearing effects of the Sun’s differential rotation and maintain their meridional shapes for several months. The answer is that a current-free corona allows the holes to follow the longitudinal deformations of the coronal magnetic field produced by large sunspot groups, analogous to the way that Earth’s tides follow the deformations of the gravitational field produced by the Moon.