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Answer: After a colon, capitalize the next word if it starts a complete sentence; lowercase it if a fragment follows.

GPO detail

• After an introductory independent clause that describes or defines what follows, capitalize the first word of a complete sentence: "His only rule was this: Chickens are not allowed past the front parlor."
• Before a final clause that extends or amplifies the preceding independent clause, lowercase the first word even if a complete sentence follows.
• A colon also introduces a run-in list: "three primary pigment colors: magenta, yellow, and cyan."

Digital.gov

• After a colon, capitalize the next word if it starts a complete sentence, not a fragment.
• In headlines, the first word after a colon is always uppercase.

See also

• plain-serial-comma-for-clarity
• plain-sentence-case-headlines

Claims

• id: C1 claim: GPO capitalizes a complete sentence after a defining clause and colon. source: S1 evidence: "If a complete sentence follows the colon, capitalize its first word."

• id: C2 claim: GPO lowercases an amplifying clause after a colon. source: S1 evidence: "Even if a complete sentence follows the colon, lowercase its first word."

• id: C3 claim: Digital.gov capitalizes after a colon only for a complete sentence. source: S2 evidence: "After a colon, capitalize the next word if it starts a complete sentence"

• id: C4 claim: Digital.gov headlines capitalize the first word after a colon. source: S2 evidence: "the first word after a colon is always uppercase in headlines"

Sources

• [S1] GPO Style Manual (2016), Chapter 8: Punctuation. U.S. Government Publishing Office (govinfo). https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2016/pdf/GPO-STYLEMANUAL-2016-10.pdf . License: US government work, public domain. Accessed 2026-11-09.

• [S2] Digital.gov style guide. Digital.gov (GSA). https://digital.gov/style-guide/ . License: US government work, public domain. Accessed 2026-11-09.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted). Sources: US federal government works (GSA, GPO, SEC, CDC), public domain in the United States; third-party quotations inside those works were not used. Text was condensed and changed from the originals.

@misc{iil_note_t36d72dp,
  author = {{iinventedlight}},
  title = {{Capitalize after a colon only when a complete sentence follows}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-09},
  url = {https://iinventedlight.com/library/notes/plain-colon-capitalization/},
  note = {Permanent ID iil:note:t36d72dp. License: CC-BY-4.0.},
}

Markdown

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