Form 870 at a glance
What it is: Form 870 (Rev. March 1992) is the Waiver of Restrictions on Assessment and Collection of Deficiency in Tax and Acceptance of Overassessment of the Department of the Treasury-Internal Revenue Service.
Who signs: If you filed jointly, both you and your spouse must sign.
A filled-in Form 870 example: a taxpayer fills the top of the form

How to fill out Form 870, box by box
Names and address of taxpayers, and the identification number
Names and address of taxpayers (Number, street, city or town, State, ZIP code).
Social security or employer identification number.
If your situation is different
You consent to the assessment
I consent to the immediate assessment and collection of any deficiencies (increase in tax and penalties) and accept any overassessment (decrease in tax and penalties) shown in Increase (Decrease) in Tax and Penalties, plus any interest provided by law. I understand that by signing this waiver, I will not be able to contest these years in the United States Tax Court, unless additional deficiencies are determined for these years.
If you consent to the assessment of the deficiencies shown in this waiver, please sign and return the form in order to limit any interest charge and expedite the adjustment to your account.
Your consent will not prevent you from filing a claim for refund (after you have paid the tax) if you later believe you are so entitled. It will not prevent the Department of the Treasury-Internal Revenue Service from later determining, if necessary, that you owe additional tax; nor extend the time provided by law for either action.
You filed jointly
If you filed jointly, both you and your spouse must sign.
The waiver is for a corporation
If this waiver is for a corporation, it should be signed with the corporation name, followed by the signatures and titles of the corporate officers authorized to sign.
An attorney or agent signs
An attorney or agent may sign this waiver provided such action is specifically authorized by a power of attorney which, if not previously filed, must accompany this form.
A fiduciary signs
If this waiver is signed by a person acting in a fiduciary capacity (for example, an executor, administrator, or a trustee) Form 56, Notice Concerning Fiduciary Relationship, should, unless previously filed, accompany this form.
The change affects your State income tax
The Department of the Treasury-Internal Revenue Service has agreements with State tax agencies under which information about Federal tax, including increases or decreases, is exchanged with the States.
If this change affects the amount of your State income tax, you should file the required State form.
The Service disallows a later claim
If you later file a claim and the Service disallows it, you may file suit for refund in a district court or in the United States Claims Court, but you may not file a petition with the United States Tax Court.
Common questions about Form 870
What is Form 870?
Form 870 (Rev. March 1992) is the Waiver of Restrictions on Assessment and Collection of Deficiency in Tax and Acceptance of Overassessment of the Department of the Treasury-Internal Revenue Service. The Department of the Treasury-Internal Revenue Service will consider this waiver a valid claim for refund or credit of any overpayment due you resulting from any decrease in tax and penalties shown in Increase (Decrease) in Tax and Penalties, provided you sign and file it within the period established by law for making such a claim.
Sources
[1] Form 870 (Rev. March 1992), Waiver of Restrictions on Assessment and Collection of Deficiency in Tax and Acceptance of Overassessment, Department of the Treasury-Internal Revenue Service (the Official source link on this page).

