What punctuation is used to join two words together in a contration?

The punctuation used to join two words together in a contraction is an apostrophe (').

For example:

* can't (contraction of "can not")

* don't (contraction of "do not")

* I'm (contraction of "I am")

* it's (contraction of "it is")

* they're (contraction of "they are")