Primary Sources are original materials or firsthand accounts of historical events. Some examples could include diaries, newspaper articles, and letters originating from the time you are studying.
More information on differentiating between primary and secondary sources can be found here.
In Quick Find, the Library of Congress Subject Headings (LCSH) for a given work are listed in the bottom half of the item record next to Subjects.
Subject headings that end in --Sources, --Correspondence, --Personal narratives, and similar are generally assigned to materials that include primary source materials.
Click on the subject heading to view all records that have been assigned to it.