Changing case is mechanical until it meets title case, at which point it becomes a question of editorial style with no universally correct answer. The other conversions are unambiguous; this page is mostly about the one that is not.
The cases
UPPERCASE. Every letter capitalised. Useful for short labels and acronyms. Avoid for anything longer than a few words: it is measurably slower to read, because the uniform shape of the words removes the outline cues readers rely on. Screen readers may also spell out all-caps words letter by letter.
lowercase. Every letter lowered. Useful for normalising data before comparison, and for URLs and email addresses.
Sentence case. First letter of the sentence capitalised, the rest left alone. The default for body text, and increasingly the house style for headings and UI labels too, because it reads faster and handles proper nouns without special-casing.
Title Case. Principal words capitalised. Conventional for headlines and book titles. Genuinely ambiguous — see below.
camelCase, PascalCase, snake_case, kebab-case. Programming conventions. camelCase for JavaScript variables, PascalCase for classes and components, snake_case for Python and SQL, kebab-case for URLs, CSS classes and filenames.
Title case has no single correct form
The major style guides disagree, and all of them are defensible:
- AP style capitalises words of four or more letters. So “Text” and “With”, but “the” and “for”.
- Chicago style capitalises all words except articles, coordinating conjunctions and prepositions, regardless of length. So “A Tale of Two Cities” — “of” stays lower however long the word.
- APA style capitalises words of four or more letters, plus all major words regardless of length.
Points of near-universal agreement: the first and last word are always capitalised, whatever they are; and subordinating conjunctions (“because”, “although”) are usually capitalised while coordinating ones (“and”, “but”, “or”) are not.
The practical advice is to pick one guide and apply it consistently. Inconsistent title case across a site looks like carelessness even to readers who could not name the rule being broken.
Where people go wrong
Losing acronyms. Converting “NASA launched the ISS module” to title case naively gives “Nasa Launched The Iss Module”. Any conversion should preserve fully capitalised words, and you should check the output when acronyms are present.
Uppercasing proper nouns away. Converting to lowercase destroys information that cannot be recovered automatically — “Polish” and “polish” are different words. Keep the original if you might need it back.
Using CSS text-transform and forgetting the underlying text. text-transform: uppercase
changes appearance only. The text copied from the page, read by a screen reader, or indexed by
a search engine is the original. That is usually what you want, but it means the visual and
the actual can diverge.
Assuming case conversion is locale-independent. It is not. Turkish has dotted and dotless i, so lowercasing “I” gives a different result in Turkish locales than in English. Rare, but a genuine source of bugs in international software.
All-caps for emphasis in body text. Use bold or italic. All-caps reads as shouting and slows everyone down.
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