Why are some hysteresis loops shaped like a butterfly?

In this paper we investigate the origins of butterfly-shaped hysteresis maps. In particular, we show that there exist 2-to-1 maps that transform oriented simple closed curves into multi-loop closed curves with loops of opposite orientations. As an illustrative application, the hysteretic input-output map of the preloaded two-bar linkage mechanism becomes a butterfly when a 2-to-1 map is applied to it. In this case, the application of the 2-to-1 map to the hysteresis loop is equivalent to plotting an alternative output variable. We also investigate the hysteretic maps of ferroelectric materials that are transformed into a butterfly through a 2-to-1 map.