Documents in last year’s Open Archives Tour focused on changing landscapes and horticultural diversity. (Photo: cambridgearchives.org)

Nine archives — private, city and Harvard-based — will open Monday through Wednesday for the third annual Open Archives Tour, delving into their collections to display rarely seen photographs and new research on Cambridge in the 1860s, said Gavin Kleespies, executive director of the Cambridge Historical Society.

“This is a unique opportunity to see some of the rare items from our collections, see the spaces where our archivists process this material and talk to the professionals who have a deep and passionate understanding of this history,” Kleespies said.

The institutions participating are the Cambridge’s City Clerk’s Office, Historical Commission, Historical Society and Public Library; Harvard University’s archives and Houghton and Schlesinger libraries; and Longfellow House’s Washington’s headquarters and Mount Auburn Cemetery.

“Cambridge is a really unusual city in terms of the number of organizations that are collecting and holding archival material, and this is an amazing chance to peek behind the scenes in nine of these institutions,” said Alyssa Pacy, of the Cambridge Room at the Public Library.

Tours on the first and third days go from 5 to 7:30 p.m.; tours on the second day go from 3 to 5:30 p.m. The programs are free, but space is limited and reservations are needed. For information on which institutions can be visited on which days, please visit cambridgearchives.org, then rsvp at rsvp@cambridgearchives.org.

This post relied heavily on a press release.