Another kind of mimicry is Müllerian mimicry, named after the widely accepted theory advanced in 1878 by the German naturalist Fritz Müller. This occurs when two species both benefit one another by looking alike because they are both equally unpalatable, as is the case with viceroy (left) and monarch (right) butterflies. Usually the species share at least one common predator.
Once you learn to tell the two apart, it's really quite easy to distinguish them. Everything else about the butterflies is nearly identical except for the viceroy's slightly smaller size and telltale bold black line that runs across its lower wings.