What Is Tertiary Source?
A tertiary source compiles or distills secondary work: encyclopedias, Wikipedia, almanacs, dictionaries, handbooks, indexes and bibliographies. It sits two steps from the evidence, which is why most rubrics bar it as citable support - while it remains the fastest legitimate way to orient yourself before the real search begins. This page unpacks the definition, the mistakes graders see most, and a worked example you can pattern-match against your own draft.
James Parker, MLS
Citation & Formatting Reviewer · Master of Library Science; 9 years as a reference librarian coaching APA, MLA and Chicago styles
Quick answer
A tertiary source compiles or distills secondary work: encyclopedias, Wikipedia, almanacs, dictionaries, handbooks, indexes and bibliographies. It sits two steps from the evidence, which is why most rubrics bar it as citable support - while it remains the fastest legitimate way to orient yourself before the real search begins.
On this page · 4 sections
The definition, precisely
A tertiary source is a compilation: it collects, condenses and reorganizes material from secondary sources without generating evidence or advancing an original argument. Encyclopedias, Wikipedia, almanacs, dictionaries, directories, handbooks, timelines, indexes and bibliographies are the standard examples - works whose job is lookup and orientation rather than interpretation. The two-steps framing is the quick test: a study reports evidence, a scholar interprets the evidence, an encyclopedia summarizes the interpretations. Most intro textbooks hover between secondary and tertiary because they summarize settled knowledge for teaching, which is why classification disputes start there and end with the question of what the source does in your paper.
Tertiary sources earn their keep in the hour before serious searching. They hand you the vocabulary a field actually uses, the key names, dates and events, the rough shape of a topic's sub-debates, and - in the case of Wikipedia and scholarly encyclopedias - a reference list pointing at the citable literature underneath. That is orientation work, and it is legitimate: ten minutes in a reference entry can save two hours of scattered searching by handing you the search terms and the names of the scholars who own the debate. The habit that separates strong students from weak ones is harvesting the references at the bottom of the entry and treating them as the actual starting line.
Why, then, do rubrics ban citing them? Three reasons hold across fields. Tertiary entries rarely name the authors responsible for specific claims, so there is no accountability chain a reader can audit. They are derivative by design - any citation to an encyclopedia is a citation to a summary of a summary, two interpretive steps from the evidence. And they are edited for stability rather than currency, so fast-moving topics outpace them silently. The practical rule most instructors mean: read tertiary sources freely for background, but never let one appear in the reference list. If a fact matters enough to cite, it matters enough to find in the secondary or primary layer.
The four mistakes graders see most
Every one of these shows up in a stack of sixty papers — usually more than once. Each pairs the error with the fix, so the table doubles as a pre-submission check.
| Common mistake | The fix |
|---|---|
| Citing Wikipedia or an encyclopedia entry in the reference list. | Keep the entry for orientation and take the citation from its footnotes instead - the studies and books it summarizes are the citable layer. If a professor spots an encyclopedia citation, the credibility cost lands on every other claim in the paper. |
| Skipping tertiary sources entirely to look rigorous. | Starting a database search with zero vocabulary produces scattered results. Read one reference entry first for the subtopics, names and terms of art - orientation is what the layer is for, and nobody grades your search notes. |
| Citing an index, bibliography or database record as if it were a source. | Those are finding tools, not sources. The citation belongs to the item the tool points to - chase the pointer, read the target, cite the target. |
| Assuming a textbook is always tertiary and always off-limits. | Textbooks are accepted for definitions and settled background in many courses and treated as secondary synthesis in others. Check the syllabus, and reserve textbook citations for claims a grader would call basic - never for load-bearing evidence. |
A worked example
A first-year student assigned a paper on gentrification in one city starts with an encyclopedia entry and the Wikipedia article on the neighborhood. Neither will be cited. From them she harvests a vocabulary list - displacement, historic preservation tax credits, demographic turnover - the names of two urban-studies scholars who dominate the debate, and six references at the bottom of the entry. Twenty minutes later she enters the library database knowing which subtopics exist, whose names to search, and which of the six references to chase first. The reference list she finally submits contains journal articles and one university-press book - and no encyclopedia.
Pattern-match your own draft against this example rather than against the definition: concrete cases transfer better than abstract rules, and the example shows what the rule looks like when it is done right.
How tertiary source differs from its neighbors
A tertiary source is not a secondary source: secondaries argue about the evidence, tertiaries compile the arguments, and the distinction is whether an author's interpretation could be disagreed with - an encyclopedia entry is designed to end conversations, not join them. It is not common knowledge either, though the two get confused: facts a general reader could be presumed to know need no citation at all, while a specific statistic from an encyclopedia entry is uncommon knowledge whose only citable version lives deeper in the layers. And it is not a primary source, obviously - it is the far end of the distance scale that the primary-source entry starts.
Boundary confusion is where most grading deductions live: the term is understood roughly but applied against the wrong neighbor. Pin the boundary above and the term stops being fuzzy in practice, which is what the rubric is actually testing.
What's your next step?
Two taps to the exact page that unblocks this paper.
The 30-second check
Skim the four mistakes above against your draft. Any hit is a fix you can make in minutes — and the format row of the rubric is where they all live.
Run it through the tool
The free mind map maker handles the mechanical half of this term — check it before you submit.
Open the toolRelated term: Peer Review — the neighbor this term is most often confused with. Reading the pair together is how the boundary sticks.
Frequently asked questions
4 answered1Is Wikipedia a tertiary source?
Yes - a volunteer-compiled summary of published work, which makes it tertiary by any definition. That does not make it useless: the reference list at the bottom is a legitimate starting point, and the articles on technical topics are often monitored closely. Use it to orient, harvest its sources, verify anything load-bearing in the original, and keep the site itself out of your reference list.
2Can I ever cite a tertiary source?
Rarely, and only when the reference work itself is the object: citing the Oxford English Dictionary for a word's etymology or a specialized encyclopedia for how a field defines a term is accepted in some courses. For ordinary claims, no - the rule is to descend one layer. When in doubt, ask the instructor; the default assumption in a rubric that says 'scholarly sources' is that encyclopedias do not count.
3Are textbooks secondary or tertiary sources?
Both labels get argued. A textbook synthesizes secondary scholarship with original pedagogy, so many fields classify it as secondary; others call it tertiary because it compiles settled knowledge for teaching rather than advancing an argument. The classification is relative to your question anyway: for a definition, a textbook citation is fine; for evidence on a contested claim, it is the wrong layer entirely.
4How do I use Wikipedia responsibly for research?
Read it for orientation: the shape of the topic, the subtopics, the vocabulary, the major names. Scroll to the references and treat that list as your real search seed. Verify anything you plan to rely on in the original source, since entry quality varies with how many editors watch the topic, and be careful on current events, where entries lag or churn. Cite the harvest, never the wiki.
James Parker, MLS
Citation & Formatting Reviewer · Master of Library Science; 9 years as a reference librarian coaching APA, MLA and Chicago styles