Historically, well-printed books have used a whole pile of symbols which have have some resemblance to a comma turned upside down, hence the title of this document. Unicode contains no less than eleven such characters. In the table below I have attempted to evoke all of them using HTML numeric entities. If you can see all eleven as distinct characters which look like their descriptions, you are fortunate.
U+0022 " QUOTATION MARK U+0027 ' APOSTROPHE U+02BB ʻ MODIFIER LETTER TURNED COMMA U+02BC ʼ MODIFIER LETTER APOSTROPHE U+02BD ʽ MODIFIER LETTER REVERSED COMMA U+2018 ‘ LEFT SINGLE QUOTATION MARK U+2019 ’ RIGHT SINGLE QUOTATION MARK U+201C “ LEFT DOUBLE QUOTATION MARK U+201D ” RIGHT DOUBLE QUOTATION MARK U+2032 ′ PRIME U+2033 ″ DOUBLE PRIME
There are also three characters which are not useful under normal circumstances. One does not normally write an accent without the letter it modifies, except as an example.
U+0060 ` GRAVE ACCENT U+00B4 ´ ACUTE ACCENT U+02DD ˝ DOUBLE ACUTE ACCENT
Proper typesetting of English text requires opening and closing single and double quotation marks, and the apostrophe. Single and double prime are also occasionally used. The apostrophe is usually the same glyph as the closing single quotation mark, but it has different spacing around it, and a computer needs to know the difference in order to find word boundaries (apostrophes can occur at the end of a word, and are logically part of it, as in the childrens' books) The turned and reversed comma are useful when typesetting languages such as Hawaiʻian, where turned comma represents a glottal stop.
Of these fourteen symbols, only three are in 7-bit ASCII,
and therefore nearly universally available without memorizing
special codes or key combinations: QUOTATION MARK,
APOSTROPHE, and GRAVE ACCENT. You will notice
that the symbol for QUOTATION MARK is what typographers
call dumb: it is a pair of vertical straight lines. This can
be used at beginning and end of a quotation, and looks acceptable as
flat text.
However, if your flat text is really marked-up input to
a typesetting program such as TeX, it's
inadequate information. Probably for this reason, some computer
systems decided to redefine APOSTROPHE as RIGHT
SINGLE QUOTATION MARK, and GRAVE ACCENT as
LEFT SINGLE QUOTATION MARK. This permits an author
to write single- or double-quoted text in plain ASCII like this:
`single quote' ``double quote''
That, plus some heuristics to tell apostrophes from closing quotes,
and special markers for when the heuristics fail, is enough to do good
typesetting of English. (Well, enough to get all the inverted commas
right.) Unfortunately, it also means that your text input looks
hideous when read with a font where GRAVE ACCENT
is really an accent grave, and APOSTROPHE is a single
vertical line. And those fonts are a growing majority.