Some municipal bonds have rallied significantly since Moody’s Investors Service and Fitch Ratings last month overhauled their systems for assigning ratings to state and local government credits — effectively upgrading almost the entire universe of muni bonds.

The effect is most noticeable for California and Puerto Rico, which enjoyed three-notch improvements in their ratings and made the symbolic leap from ­ratings beginning with the letter “B” to those beginning with an “A.”

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