This fund aims to replicate the investment performance of the S&P AMT-Free Municipal Series Dec 2021 Index. This benchmark is composed of investment-grade, non-callable U.S. municipal bonds that reach maturity in 2021. Typically, the fund commits at least 90% of its capital to the constituent securities of this underlying index. A maximum of 10% of its assets may be allocated to financial derivatives like futures, options, and swap agreements, or to highly liquid assets such as cash and cash equivalents, which can encompass money market funds managed by BFA or its affiliated entities. This 10% portion may also include securities not present in the primary index, if BFA determines they contribute to the fund's objective of tracking the index.