What gets straightened
Each curly glyph is its own Unicode code point. A parser sees a different character, not a prettier version of the ASCII key.
Double quotes: the opening and closing curly pair (U+201C, U+201D).
Single quotes and the apostrophe: U+2018 and U+2019. The apostrophe costs people the most time, because a curly one inside a contraction breaks a string literal and is nearly invisible in most fonts.
Prime marks: U+2032 and U+2033, which measurements in Word documents and PDFs leave behind.
Low-9 quotes and guillemets: the German-style pair (U+201A, U+201E) and the French guillemets (U+00AB, U+00BB, U+2039, U+203A), so quoted text out of a European document comes back plain.
Ellipsis: U+2026 becomes three periods. The toggle is on by default, since that glyph breaks the same pipelines the quotes do.
Why straight quotes matter
Every programming language treats only straight quotes as string delimiters. Python, JavaScript, SQL and shell all reject the curly versions, so a snippet that looks correct on screen fails the moment it runs. The error is usually a blunt SyntaxError: invalid character, and you can stare at the line for a minute before spotting the curve.
This does not run:
print(‘hello world’)
Straighten the two quotes and it does:
print('hello world')
JSON is stricter. The specification requires straight ASCII double quotes around keys and string values, so one curly quote turns a whole file into a parse error. CSV imports, command-line arguments and config files break the same way.
They come back
Word, Google Docs, Pages and Outlook convert straight quotes as you type, under a setting called smart quotes. Phone keyboards do the same. So does macOS text substitution, system-wide. Copy a code sample out of a doc, a slide or a PDF and the curly marks ride along.
The awkward part is that they return on the next edit or paste. A cleanup here fixes the text in front of you and nothing after it. On a Mac, Klipto straightens quotes as part of the paste, so the version that lands in your editor is already plain.