The benchmark S&P 500 could flirt with a bear market on Monday, a 20% drop from its recent high. It would need to drop 3.1%—though stock-index futures are currently pointing to a shallower drop.

Another data point: If the S&P 500 drops more than 3.8% today, combined with last Thursday and Friday, it will have fallen 14%. That would be the biggest such plunge since a 26.4% three-day wipeout around October 1987’s Black Monday.

The worst fall over three sessions during the global financial crisis was a 13.9% drop in October 2008.