IRS Publication 501 — Dependents, Standard Deduction, and Filing Information
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You can change your filing status from a separate return to a joint return by filing an amended return using Form 1040-X.
You can generally change to a joint return any time within 3 years from the due date of the separate return or returns. This doesn’t include any extensions. A separate return includes a return filed by you or your spouse claiming married filing separately, single, or head of household filing status.
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