"The CBOE VIX and VXN index prices both are designed to reflect the implied volatilities of certain index options contracts; VIX is based on the prices of eight S&P 100® (OEX®) index puts and calls, while VXN is based on the prices of Nasdaq-100® (NDXSM) options" http://www.cboe.com/micro/vixvxn/introduction.asp