Tertiary sources compile, summarize, index, or help identify scholarly sources on a topic. They are useful for background research or to get an overview of a topic before delving into secondary or primary sources. Common tertiary sources for City and Regional Planning include dictionaries, encyclopedias, handbooks, and other reference sources.
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The following two encyclopedias will give you background information and definitions of important terms in City and Regional Planning. Both encyclopedias can be accessed online.
Encyclopedic Dictionary of Landscape and Urban Planning by 
Encyclopedia of the City by 
