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@Accountant-Man wrote:

What does it matter? Married equals MFJ or MFS, but special circumstances allow HOH.

Not married equals S, HOH or Qualified Widow/er.

You're either married, or you're not married. 

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It depends.

For example, the Earned Income Credit is disqualified for MFS.  UNLESS they live apart for the last six months.  And there are several other similar provisions that are based on if they live together or live apart.

That is why I asked for the specific tax circumstances that benefit from living apart.