The 26 topologically different perestroikas of the momentary shock waves are shown by arrows (after Bogaevski).
Those relevant to the Medial Axis are indicated by a double-arrow.
Single lines indicate the (exterior) boundary of the shock illustration.
Double lines indicate rims where shock sheets are limited by surface ridges, i.e., they correspond to A_3 shock curves.
Triple lines indicate intercepts of shock sheets, i.e., they correspond to A_1^3 shock curves.
Concentric circles indicate an overlap of medial points.