The investment seeks to track the investment results of the S&P AMT-Free Municipal Series Dec 2021 IndexTM, which measures the performance of investment-grade, non-callable U.S. municipal bonds maturing in 2021. The fund generally will invest at least 90% of its assets in the component securities of the underlying index, and may invest up to 10% of its assets in certain futures, options and swap contracts, cash and cash equivalents, including shares of money market funds advised by BFA or its affiliates, as well as in securities not included in the underlying index, but which BFA believes will help the fund track the underlying index.