Analysis of the observables in three images d)–f) determines whether to apply the fold or cusp equations and where the source is located with respect to the folds and cusp a)–c): a fold configuration a) and e) shows two highly symmetric images, A, B, with larger separation to the third image, C, less magnified than A, B. In a cusp configuration b) and e), c) and f), A is highly magnified compared to B and C because it can be considered an overlay of two images generated by lensing at the upper and the lower fold which unite at the cusp point. Image shapes are neglected in these graphs, but high and low absolute values of magnifications are indicated by dark and light grey values, respectively.