Black Monday 1987
The collapse of the US market in October 1987 — the largest single-session fall on record.
How to read the number
Portfolio insurance amplified it: programmes sold futures automatically as prices fell, the selling pushed prices lower, and the lower price triggered more selling. Exchange-wide halt rules followed.
When the metric lies
No convincing news cause has ever been found, which is the lesson: trading mechanics alone can move a price without an outside trigger.
Also known as: black monday