The apostrophe is an omission mark. The correct apostrophe is in the form of a 9 (with a filled stamp) or the comma of the respective font.
The apostrophe marks omissions of letters or parts of words in a word.
Not a single (single) bite anymore
Ku'damm (Kurfürstendamm)
The apostrophe also clarifies the genitive of proper names without an article. This usually happens with an ending in “s”, but also after the letters ß, ß, tz, z, x and ce.
The red one is Klaus' cup.
Marx's theories are highly controversial.
More typography tips and basics can be found in the book”The ABC of typography”.
