Which 1987 index suffered the world's largest one-day percentage fall during Black Monday?

The Dow Jones Industrial Average suffered the world's largest one-day percentage fall during Black Monday in 1987.

On 19 October 1987, the Dow dropped 508 points, equal to 22.6% of its previous closing value. The decline was measured in percentage terms, which is important because a later point loss can be numerically larger without representing as large a proportion of the index. The Dow tracks 30 major U.S. companies and is price-weighted rather than market-capitalization-weighted.

The S&P 500 and Nasdaq Composite also fell sharply that day, but neither recorded the same percentage decline as the Dow. International indexes had already dropped heavily as the trading day moved westward around the globe.

The crash prompted investigations into market structure, program trading, and portfolio insurance. It also helped lead to coordinated emergency procedures and circuit breakers. The 1987 event therefore affected both the historical record and the later design of stock-market trading systems.