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For example, if Harry and Sally own a home as tenants in common, and they are not married, upon Harry's death his heirs will inherit his ownership interest, not Sally. Of course, exceptions can and do exist. So it is wise to consult with an attorney and not rely on a blog....this is not legal advice.
It never fails, several times a year I have clients who are amazed to read the deed to their home and discover phrasing such as tenants in common. Even if married, the interest would have to pass through probate to go to the spouse.