Recto: The front side of a page.

Verso: The back side of a page.

Age-toning: A descriptive word for areas of discoloration or browning that have appeared on old maps over the years. Generally from improper storage and handling.

Blue-Back chart: A method used by private publishers in London during the 19th century to distinguish them from the official Admiralty charts. The name came from the heavy blue paper backing used by publishers to strengthen the charts. The blue-backing was also used by American chartmakers Edmund Blunt and George Eldridge in the late 19th/early 20th century.

from http://www.gracegalleries.com/Glossary_of_Map_Terms.htm