A CP12 notice, also known as a Math Error Notice, is sent by the IRS when there is an arithmetic or clerical error on your tax return. It outlines the specific error and provides a revised tax calculation. To respond, you should carefully review the notice and the proposed changes. If you agree with the changes, you do not need to take any action, and the IRS will adjust your tax liability accordingly. However, if you disagree with the changes, you must provide a written explanation and supporting documentation within the specified timeframe, typically 60 days. Ensure your response is timely and complete to avoid further interest and penalties. Reference the relevant sections of the Internal Revenue Code, such as §6213(b) for math error adjustments.