There is a level at which selloffs in U.S. equities have rapidly morphed into rallies in the last five months: 1,900 on the Standard & Poor’s 500 Index.

The benchmark gauge for American stock has taken three trips below this threshold on three occasions only to vault back out via single-day rallies averaging 2.5 percent. Most recently, the S&P 500 gained 1.7 percent Thursday after it closed at 1,890.28. That mimicked August and September, when the gauge rallied 3.9 percent and 1.9 percent, respectively, after penetrating that low.

U.S. stocks slumped below 1,900 Friday, falling 2 percent to 1,883.78 as of 9:53 a.m. in New York, as oil’s plunge past $30 sent markets reeling after Chinese shares tumbled into a bear market.

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