He has been working ________ several years on this theory.

  • a) off and on
  • b) on and off

It sounds to me ridiculous but my book says it should be "off and on". Aren't they interchangeable? And if not, what are the nuances?

Thanks in advance.


They are interchangeable, and this ngram view suggests that on and off is currently about three times as common as off and on. Off and on was a bit more common before the beginning of the 20th century. So perhaps your book is very old.

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