Citing sources correctly

Fact-checked by Benjamin Lohrer, editor at corrected.eu · Last updated: 28 September 2026

Citing sources correctly: books, websites, AI

A source citation is more than a duty at the end of a sentence. It makes your work traceable, protects you against accusations of plagiarism and shows your examiner that you have worked carefully. This guide explains how to cite the most common types of source (books, journal articles, websites, AI tools) and which pitfalls to avoid. For the formal details of APA style, see our APA 7 guide. If you are short of time, the citation generator creates the reference automatically from a DOI, ISBN or URL.

What must a source citation achieve?

A correct citation has to do three things: show where the information comes from, allow the reader to trace it back to the original source, and clearly separate borrowed material from your own argument. If you cover all three, you are on the safe side, even though the exact format differs between citation styles. You can find an overview of the styles under citation styles.

Citing a book (monograph)

For a book you need the author, year of publication, title (in italics), edition (if not the first) and publisher. Example in APA 7:

In the text: (Schmidt, 2022, p. 47).

Journal article

For a journal article you need the author, year, article title, journal name (in italics), volume(issue), page range and DOI. Example:

Chapter in an edited book

An edited book contains several independent chapters by different authors. When you cite one of them, give the name of the chapter's author and the year first, followed by the chapter title. Then list the editors of the whole book (with "Ed." or "Eds."), the book title in italics, the page range of the chapter and finally the publisher. A common mistake is to cite only the whole book. That leaves the reader guessing which part you mean.

Online material

Websites and digital documents follow clear rules. First identify the person or organisation responsible. Add the exact publication date, then the page title in italics, and finally the web address. If the content changes over time (as with encyclopaedias), record the date on which you retrieved it. A concrete pattern:

PDF documents

PDFs are treated like books or web sources, depending on how they were published. An official PDF has the authority as its author, and a scholarly working paper has its authors. For PDFs that stay online for the long term, the URL is enough without a retrieval date.

Video, podcast, YouTube

For video and audio sources you need the uploader or speaker, date, title (in italics), platform and URL. On YouTube the "uploader" is the channel name, and for podcasts it is the host. In some disciplines (media studies, film studies) video sources are accepted as scholarly sources. In others they should be used sparingly and always with a reason.

Social media

Posts on X, LinkedIn or Instagram can serve as sources as long as they are public. The format is: author [username], date, title or first words, [post type], platform, URL. If a post is private or has been deleted, the source can no longer be verified. In that case it is better not to cite it, or to include a screenshot in the appendix.

Generative AI

Generative systems such as large language models need to be clearly flagged. In APA 7 the developer is the author, and the output is not retrievable by others. The pattern is: Developer. (Year). Name of tool (version date) [Type of model]. URL. In the text you cite it briefly as (Developer, Year). Because such tools produce different answers each time, an exact repetition is impossible. That makes them poor primary evidence. Check the rules of your university and your supervisor first.

Statute, court decision, regulation

Legal sources have their own conventions. For statutes, the abbreviation (for example "BGB § 433") is enough in the text, and a full reference is not needed in the bibliography. For court decisions, give the court, date and case number. Example: BGH, judgment of 15 May 2020, I ZR 124/19. Always follow the legal citation guidelines of your faculty.

Secondary citation: when you do not have the original

Sometimes the original is not available. In that case you cite it as a secondary source: (Müller, 1980, as cited in Schmidt, 2022, p. 47). The bibliography contains only the source you have actually read, in this case Schmidt 2022. Secondary citations should remain the exception. If the original is available, read it.

In the text vs. in the bibliography

In the text you give the short form: (Schmidt, 2022, p. 47). The bibliography contains the full reference. Both must be consistent: identical author names and an identical year. A common source of error is a text citation "(Schmidt 2022)" next to a bibliography entry "Schmidt, Andreas (2023)". Inconsistencies like these are regularly found in editing. How to set up the list itself is covered in our guide to the bibliography.

Common mistakes in source citations

• A source appears only in the text but not in the bibliography (or the other way round).
• The place of publication is given, although APA 7 no longer requires it.
• The DOI has a "doi:" prefix instead of being a full URL.
• For three or more authors, the text names the second author instead of using "et al." (APA 7).
• In-text citation and bibliography entry do not match.
• Web sources are given without a date although a date exists.

Tools that make it easier

Three widely used tools: Word with its built-in source manager, Citavi (often licensed for free at German universities) and Zotero (open source). Whichever tool you use, a manual spot check is a must, especially for edited books and non-English journals.

For more help with specific source types, see citing a dissertation, the templates and downloads and the citation check.