Putney, Twombley, Hall & Hirson, one of New York City’s oldest law firms, has signed on for another 10 years at 521 Fifth Ave., its home for two decades. The firm, founded in 1866, has nearly 21,000 square feet—the entire 10th floor of SL Green Realty Corp.’s 39-story building.

The 459,850-square-foot tower was built in 1929 and stands on the northeast corner of Fifth Avenue and East 43rd Street.