whatForty-eight banks in a ring, each lending a fifth of its balance sheet in equal slices to Z counterparties, over a thin capital buffer. When a bank fails, everyone who lent to it loses that slice; whoever's losses pass the buffer fails next, and the dominoes go round. Failed banks are struck through, the arcs that carried losses flash, and every shock's final toll lands on the chart against Z.

whyThe claim: the same shock fizzles or takes the system depending only on the wiring. Measured here, one failed bank takes a third of the system at one counterparty, nine tenths at three, all of it at four -- and then the cliff: at five or more, each slice is smaller than the buffer, no single loss can kill anyone, and the toll drops to exactly the one shocked bank, forever. Danger versus connectivity is not a slope but a mountain with a cliff, which is why regulators argue about interconnection in both directions at once.

howGai and Kapadia's contagion model in miniature: interbank share fixed at twenty percent, buffer at four and a half, losses propagate in waves with full loss-given-default. Slide the counterparties and the web rewires; click any bank to shock it yourself; left alone the system shocks one at random. One integer seeds who lends to whom.