The library catalog uses Library of Congress Subject Headings. The list of authorized subject headings is available online. However, you can start with keyword searches to gather some relevant titles, then look at their subject headings to discover the official terms to use.
Here are some subject headings related to Medieval history:
Aesthetics, Medieval
Archaeology, Medieval
Architecture, Medieval
Armor, Medieval
Art, Medieval [Also various specific types of art,
e.g., Altarpieces, Medieval
Artists, Medieval
Astronomy, Medieval
Authors, Medieval
Biography -- Middle Ages, 500-1500
Bookbinding, Medieval
Bridges, Medieval
Chivalry
Christian sects, Medieval
Church history -- Middle Ages
Cities and towns, Medieval
Civilization, Medieval
Cookery, Medieval
Cosmology, Medieval
Criticism, Medieval
Drama, Medieval
Education, Medieval
Ethics, Medieval
Etiquette, Medieval
Europe -- History -- 476-1492
Feudalism
Fourteenth Century
Funeral rites and ceremonies, Medieval
Geography, Medieval
Knights and knighthood
Literature, Medieval
Manuscripts, Latin (Medieval and modern)
Manuscripts, Medieval
Marriage customs and rites, Medieval
Medicine, Medieval
Medieval music
Medievalism
Middle Ages
Military art and science -- History -- Medieval, 500-1500
Military history, Medieval
Nobility -- Education, Medieval
Philosophy, Medieval
Rhetoric, Medieval
Rites and ceremonies, Medieval
Satire, Medieval
Science, Medieval
Tales, Medieval
Thirteenth Century
Tournaments, Medieval
Travel, Medieval
Women -- Education, Medieval
[Subdivision] History -- To 1500
More on Subject Searching:
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Even if you are doing a keyword search, using phrases from the subject headings typically improves the relevance of your results.
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As you search subject topics in the catalog and WorldCat, note that the following subdivisions indicate primary source material: correspondence
diaries
interviews
personal narratives
pictorial works
songs and music
sources
speeches, addresses, etc.
treaties
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It can be hard to guess the order of subdivisions and to retrieve all relevant headings. You can, however, do "power searching" in the Advanced Search Options. See the Help page for more details. Example: d:Medieval and d:Sources retrieves "Social history -- Medieval , 500-1500 -- Sources " and "Great Britain -- History -- Medieval period, 1066-1485 -- Sources