Theodora. Detail from the 6th-century mosaic "Empress Theodora and Her Court" in the Basilica of San Vitale in Ravenna..
Reference works are authoritative resources that provide facts and/or summaries of information. They should be used in your research process to gain a quick overview or summary of your topic (encyclopedias); define unfamiliar terms (dictionaries); and find geographic information (atlases and maps). Many U.S. government websites provide reference resources with statistics for quantitative support of your research topic arguments. Reference works are usually considered Tertiary Resources. They are used as a jumping off point for further research.