Are double contractions formal? Eg: "couldn't've" for "could not have"

Are double contractions, such as following, formal (ie allowed in formal documents/papers)?

• it'll've for "it shall have" or "it will have"


• mightn't've for "might not have"

How about multiple contractions such as y'all'd've for "you all would have" ?

Answer

No, double or multiple contractions are not formal.

While some style guides support the moderate use of common contractions, even single contractions are sometimes discouraged in formal writing. See MLA style on contractions and this roundup of views on contractions.

Edit to address some of the points in the comments:

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  In formal writing, it is appropriate to use contractions if you are quoting a line of text or speech that contains contractions, or if the topic you are discussing is the use of contractions.

  
  


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  O'clock is standard and formal.

  
  


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  Diacritical marks in words like Qur'an, Hawai'i and Xi'an are not contractions and are not discouraged.