The CBOE Volatility Index—also known as the VIX—is a primary gauge of stock market volatility. The VIX volatility index offers insight into how financial professionals are feeling about near-term market conditions. Understanding how the VIX works and what it’s saying can help short-term traders tweak their portfolios and get a feel for where the market is headed.

What Is the VIX?

The VIX, formally known as the Chicago Board Options Exchange (CBOE) Volatility Index, measures how much volatility professional investors think the S&P 500 index will experience over the next 30 days. Market professionals refer to this as “implied volatility”—implied because the VIX tracks the options market, where traders make bets about the future performance of different securities and market indices, such as the S&P 500.

For people watching the VIX index, it’s understood that the S&P 500 stands in for “the stock market” or “the market” as a whole. When the VIX index moves higher, this reflects the fact that professional investors are responding to more price volatility in the S&P 500 in particular and markets more generally. When the VIX declines, investors are betting there will be smaller price moves up or down in the S&P 500, which implies calmer markets and less uncertainty.

It’s important to note here that while volatility can have negative connotations, like greater risk, more stress, deeper uncertainty or bigger market declines, volatility itself is a neutral term. It’s simply a statistical measure of price changes for a security or an index. Greater volatility means that an index or security is seeing bigger price changes—higher or lower—over shorter periods of time.

How Does the VIX Measure Market Volatility?

The VIX index measures volatility by tracking trading in S&P 500 options. Large institutional investors hedge their portfolios using S&P 500 options to position themselves as winners whether the market goes up or down, and the VIX index follows these trades to gauge market volatility.

The options market can be somewhat opaque, but technically speaking, the VIX measures volatility by looking at strike prices related to different puts and calls based on options contracts that expire in one month as well as those that expire on different Fridays of the upcoming month. Prices are weighted to gauge whether investors believe the S&P 500 index will be gaining ground or losing value over the near term.

Generally speaking, if the VIX index is at 12 or lower, the market is considered to be in a period of low volatility. On the other hand, abnormally high volatility is often seen as anything that is above 20. When you see the VIX above 30, that’s sometimes viewed as an indication that markets are very unsettled.

How Can You Invest in the VIX?

There are a range of different securities based on the CBOE Volatility Index that provide investors with exposure to the VIX. Alternatively, you can buy and sell VIX options and futures contracts.

Perhaps the most straightforward way to invest in the VIX is with exchange-traded funds (ETFs) and exchange-traded notes (ETNs) based on VIX futures. As exchange-traded products, you can buy and sell these securities like stocks, greatly simplifying your VIX investing strategy.

One of the most popular and accessible of these is the ProShares VIX Short-Term Futures ETF (VIXY), which is based on VIX futures contracts with a 30-day maturity. Some exchange-traded securities let you speculate on implied volatility up to six months in the future, such as the iPath S&P 500 VIX Mid-Term Futures ETN (VXZ), which invests in VIX futures with four- to seven-month maturities.

Before investing in any VIX exchange-traded products, you should understand some of the issues that can come with them. Certain VIX-based ETNs and ETFs have less liquidity than you’d expect from more familiar exchange traded securities. ETNs in particular can be less liquid and more difficult to trade as well as may carry higher fees.

Making Investment Decisions Based on the VIX

Market professionals rely on a wide variety of data sources and tools to stay on top of the market. The VIX is one the main indicators for understanding when the market is possibly headed for a big move up or down or when it may be ready to quiet down after a period of volatility.

Experts understand what the VIX is telling them through the lens of mean reversion. In finance, mean reversion is a key principle that suggests asset prices generally remain close to their long-term averages. If prices gain a great deal very quickly, or fall very far, very rapidly, the principle of mean reversion suggests they should snap back to their long-term average before long.

The VIX index tracks the tendency of the S&P 500 to move away from and then revert to the mean. When the stock markets appear relatively calm but the VIX index spikes higher, professionals are betting that prices on the S&P 500—and thereby the stock market as a whole—may be moving higher or lower in the near term. When the VIX moves lower, investors may view this as a sign the index is reverting to the mean, with the period of greater volatility soon to end.

As an investor, if you see the VIX rising it could be a sign of volatility ahead. You might consider shifting some of your portfolio to assets thought to be less risky, like bonds or money market funds. Alternatively, you could adjust your asset allocation to cash in recent gains and set aside funds during a down market.

On the other hand, during times when the VIX is falling, indicating the possibility of more stability to come in the stock market, it might make more sense to focus on individual stocks or other riskier assets that might fare well during times of growth.

Just keep in mind that with investing, there’s no way to predict future stock market performance or time the market. The VIX is merely a suggestion, and it’s been proven to be wrong about the future direction of markets nearly as often as it’s been right. That’s why most everyday investors are best served by regularly investing in diversified, low-cost index funds and letting dollar-cost averaging smooth out any pricing swings over the long term.