I don't get this:
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In addition to nonagency MBS, the other portion of PTIAX’s “barbell” is longer-duration, tax-exempt municipal bonds. In the event of a continued rise in interest rates, the municipal portion of PTIAX should provide a relative volatility “buffer.”
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In the event that rates rise, wouldn't the long duration portion of the barbell suffer the greatest losses? Maybe the short end of the barbell has negative duration, and the long end is a buffer if rates continue to fall?