Brattle Hall is a historic building along Brattle Street near Harvard Square in Cambridge, Massachusetts. It was constructed in 1889 for the Cambridge Social Union – established in 1871 – when that organization moved into the adjacent William Brattle House that year. Brattle Hall was built to house the organization's library, and to provide a space for larger meetings and social functions. Brattle Hall was designed by Longfellow, Alden & Harlow, originally in the Dutch Colonial Revival style, but it acquired more of a Colonial Revival feel with the 1907 addition of brick ends, designed by Charles Cogswell.