The club opened Union Park, sometimes called Oriole Park, and retroactively designated Oriole Park III, on May 11, 1891 also south of Waverly at Greenmount Avenue and Sixth Street (also called Huntingdon Avenue, and today known as 25th Street). The team joined the National League in 1892 after the American Association folded. Union Park was the Orioles' home when the club won three straight National League pennants and the championship Temple Cup twice. Baltimore was dropped when the National League contracted from 12 to eight teams in 1900.

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