RV curves of pairs of emission peaks from different ions in γ Cas as labeled. Initially, the peaks were widely separated. Later, they approached each other until they became unresolved in the single-line stage, after which they split again. If the curves crossed during the single-line stage, the peaks retained their relative strengths (which during that time were very different) but the signs of the RVs were swapped, which means that matter initially moving toward the observer moved away from them after the crossing (and vice versa). If the curves merely temporarily touched one another, the relative strengths were swapped whereas the signs of the RVs remained the same (figure reproduced from Heard 1938).