The Vanguard Tax-Exempt Bond Index ETF employs an indexing investment approach designed to track the Standard & Poor's National AMT-Free Municipal Bond Index, which measures the performance of the investment-grade segment of the U.S. municipal bond market. This Index includes municipal bonds from issuers that are primarily state or local governments or agencies whose interest is exempt from U.S. federal income taxes and the federal alternative minimum tax (AMT). To be eligible for inclusion in the Index, each bond must have a rating of at least investment-grade, as determined by a nationally recognized statistical rating organization (e.g., at least BBB- by Fitch Ratings, Inc.); be denominated in U.S. dollars; and have a minimum par amount of $25 million. In addition, to be included in the Index, each bond must have a minimum term to maturity or call date greater than one calendar month.