Brattle Hall is a historic building at 40 Brattle Street in Harvard Square, Cambridge, Massachusetts. The Colonial Revival building was constructed in 1889 for the Cambridge Social Union to a design by Longfellow, Alden & Harlow. The Union was established as a social club in 1871, and moved into the adjacent William Brattle House in 1889. The hall was built to house the organization's library, and to provide a space for larger meetings and social functions. Original Dutch Colonial in character, the building acquired more of a Colonial Revival styling with the addition in 1907 of the brick ends, as designed by architect Charles Cogswell.