Vanguard Mid-Cap Index Fund Admiral Shares 

This low-cost index fund provides broad mid-capitalization U.S. equity exposure. The fund seeks to track an index of medium-sized companies, whose stocks tend to be more volatile than those of larger companies. This potential volatility is one of the fund’s key risks. Investors looking to add a passively managed, mid-cap stock allocation to an already diversified portfolio may wish to consider this fund.