Traditionally, reference materials provide a starting point for research, providing background information, definitions, statistics and paths to other sources of information. They tend to be introductory and narrow in focus - if your topic is very recent or or specific, reference material may not prove helpful.

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Reference materials include:

Material Definition:
Almanacs Collection of dates, facts and figures
Atlases Collection of maps and geographic information
Biographies Profiles of notable figures
Directories List of associations, companies and institutions
Encyclopaedias Introductory topic information such as a definition, description, and brief history
Year books and annuals Annual collection of facts, photographs or statistics.

Typically, print reference materials are large heavy books often in multiple volumes. For many disciplines printed reference material has become less popular as electronic editions become available. For a list of electronic reference resources, see the library databases subject guide listed by resource type.

Wikipedia, the online user generated encyclopedia, is freely available and often a good place to do some initial reading however it is best not to rely on it as it would NOT be considered an appropriate source for most academic writing.

For more information on sources see Evaluating information. For more on Wikipedia see 1.7.8.1 Social media.