Plural of “thesis”

The plural of thesis is theses.

Why

"Thesis" comes from Greek "thesis," meaning "a placing" or "a proposition put forward," and English kept the Greek -is/-es plural pattern, the same one governing "analysis/analyses" and "crisis/crises": the plural adds a full extra syllable, pronounced "THEE-seez," not simply an added s sound.

Because "thesis" is used almost exclusively in academic and formal argumentative writing, it has shown essentially no drift toward a regularized "thesises" the way more everyday Latin and Greek borrowings sometimes do, reinforced by how consistently the entire -is/-es family (crisis, analysis, hypothesis) behaves the same way across academic vocabulary a writer encounters constantly in that register. Doctoral programs in different countries use "thesis" and the related term "dissertation" somewhat differently — in the United States, "dissertation" typically refers to the doctoral-level document while "thesis" is reserved for master's-level work, whereas in the United Kingdom and much of the Commonwealth, "thesis" often covers both levels, a genuine terminology difference layered on top of, but separate from, the shared plural pattern both words happen to follow differently, since "dissertation" pluralizes fully regularly.

Usage note

The word carries two closely related senses that both pluralize identically: a formal research paper submitted for a degree, and, more generally, the central argument or claim of any piece of writing ("the thesis of her essay") — both meanings use "theses" for the plural without any distinction in form.

"Antithesis," a compound built on "thesis" meaning the direct opposite of something, follows the exact same plural pattern ("antitheses"), extending the Greek pattern into one of the more common abstract nouns in philosophical and rhetorical writing, distinct from but closely related to the base word's own academic sense. University writing centers commonly include a specific note on the thesis/theses distinction in their style guides for graduate students, since it's one of the most frequently corrected errors in early drafts of academic work submitted by students still unfamiliar with formal academic register. Some fields, particularly the fine arts, use 'thesis' loosely for a culminating creative project rather than a strictly written argument, a genuine extension of the word's meaning that still follows the identical plural pattern as its more traditional academic sense.

Common real mistakes

Pronouncing the plural "theses" the same way as the singular "thesis" is a common spoken error among learners, since nothing in the spelling clearly signals the extra syllable the plural actually requires; in writing, the far more common mistake is simply forgetting the plural exists and writing "thesises," a straightforward overregularization. A further common error among students: using "thesis" itself as a verb ("to thesis about something"), an informal coinage that has no standing in formal writing and should be replaced with a proper verb like "argue" or "propose."

The wider pattern

Grammatically, "thesis/theses" runs with the same Greek -is/-es crowd as "analysis/analyses," "crisis/crises," "hypothesis/hypotheses," and "parenthesis/parentheses" — a large, mutually reinforcing family of academic vocabulary that has stayed remarkably stable precisely because so many related words keep the identical pattern alive together.

Examples

• She spent two years researching her doctoral thesis.
• The professor reviewed several theses before the defense season began.
• His thesis argued that the policy had unintended consequences.
• Both theses reached remarkably different conclusions from similar data.
• The committee approved her thesis after minor revisions.
• Graduate students defend their theses in front of a faculty panel.
• A strong thesis statement anchors the entire essay.
• The library holds decades of unpublished master's theses.
• Reviewers compared the two theses for originality and rigor.
• His thesis challenged a long-held assumption in the field.
• Her thesis advisor suggested narrowing the topic before the next draft.
• The library keeps bound copies of every doctoral thesis submitted since 1960.
• The committee asked for substantial revisions before accepting the thesis.