Start with the question you are asking
Online research often begins with a search result, not a library shelf. A photograph, report, encyclopedia entry, and blog post may appear side by side. To sort them, start with your research question. A source is primary, secondary, or tertiary according to the job it does for that question. Its format alone does not settle the matter.
Suppose you want to know what a town council decided about a park. The meeting minutes may be primary evidence of the recorded decision. A local writer’s article explaining the debate is a secondary source. A general page summarizing the town’s history may be tertiary. Change the question to how local writers covered the decision, and the article itself becomes primary evidence. Before assigning a label, finish this sentence: “I am using this source to learn about ______.”
Primary sources: Evidence close to the subject
A primary source gives you direct material to examine. Depending on the topic, it might be a letter, interview recording, dataset, photograph, original study, public document, or firsthand account. Online access does not make it less primary: a scan of a letter can still let you study the letter’s wording. But access can affect what you can verify. A cropped image, missing page, or transcript without its recording may leave out important context.
Ask who created the material, when and why they created it, and whether the page shows the whole item. A firsthand account offers a person’s perspective, not a complete view of an event. Likewise, a dataset can give you measurements without explaining every choice behind collection. Primary does not mean automatically accurate or unbiased; it means the source is close to what you are investigating.
Secondary sources: Interpretation and analysis
A secondary source examines, explains, compares, or argues about primary material. An article that interprets council records, a book chapter comparing several letters, or a review discussing the findings of multiple studies may serve this role. Secondary sources can help you understand context and notice questions you would not have thought to ask of the original material.
Look for signs that the writer makes an interpretation: they select evidence, connect events, explain a pattern, or challenge another account. Then check whether you can see what supports their conclusions. Links, citations, named documents, and clear descriptions of methods make an argument easier to inspect. An opinionated page can still be useful, but separate its claims from the evidence it presents. If a striking claim rests on one linked document, open that document and see whether it supports the wording.
Tertiary sources: A map of the topic
Tertiary sources collect and condense information from other sources. Encyclopedias, glossaries, subject guides, and some reference timelines often work this way. They are useful when you need definitions, names, search terms, or an outline of an unfamiliar subject. A reliable overview can point you toward the records and analyses needed for a closer look.
Treat an overview as a starting map rather than the destination for every question. Check whether it identifies its contributors and shows where its information came from. Some pages mix source types: an overview may include an original interview, while an archive page may add a curator’s interpretation beside a scanned document. You can classify the particular part you use instead of forcing the entire website into one category.
A short routine for checking online sources
When you find a useful page, take three steps. First, name your question and decide whether the page offers direct evidence, interpretation, or a summary. Second, trace its origin: who created the content, where is the original item hosted, and does a link lead to the full document rather than another summary? Third, note any limits, such as an undated copy, an excerpt, or a claim you cannot trace. Search snippets and reposts can help you find material, but they may omit the context you need.
Try the routine with one topic you are curious about. Find an overview to learn the vocabulary, an analysis that addresses your question, and one piece of direct evidence the analysis uses. Write a sentence describing what each source can—and cannot—tell you. The goal is not to collect one of each type for every project. It is to know what kind of answer each source can reasonably support.
