The Denver Public Library has been collecting material on Western history and genealogy since the first several decades of the 20th century, and their genealogy materials comprise the largest such holdings in the Rocky Mountain area. For persons interested in utilizing their services, their site contains useful materials about their various topical holdings, such as primary manuscripts, photographs, and newspapers. One particular online resource is the photograph archive, which contains over 100,000 photographs which feature images of pioneer life, mining, Denver, and railroads. Visitors are encouraged to search this archive by geographic location, subject, personal name, or date. Another interesting feature is the "Building Histories" tutorial, which was designed to teach individuals how to research the history of various structures through photographs, building permits, and atlases.