Volatility Index (21 Jun 13): $VIX 18.90

The CBOE’s Volatility Index, known as the VIX, closed above 20 Thursday for the first time this year, a move that coincided with the S&P 500′s biggest back-to-back decline since 2011. The VIX has since pulled back, recently down 9.4% to 18.56.

The VIX typically moves inversely to stocks, with that correlation stronger when stocks fall, because a drop in stocks usually sparks buying of options looking to protect against–or profit from–heightened price swings. The index is calculated from the prices investors are willing to pay for options tied to the S&P 500.