The Sanborn Fire Insurance Maps are an amazing resource for anyone interested in local history and urban planning. In the period between the 1860's and 1970's, the Sanborn Company created maps of "more than 12,000 American towns and cities. They show the size, shape and construction materials of dwellings, commercial buildings, factories and other structures. They indicate both the names and width of streets, and show property boundaries and how individual buildings were used." (Library of Congress press release, 5/25/2017).
The Library of Congress obtained the copyright to these maps and has been digitizing them. Now, over 25,000 are available online in their Sanborn Collection, and more are being added all the time. Their "About this collection" page explains the keys, legends, and color schemes, as well as tips on interpreting scale and the indexes.