If the IRS determines that an employee does not have enough withholding, the IRS will notify an employer to increase the amount of withholding tax by issuing a “lock-in” letter that specifies the maximum number of withholding allowances permitted for the employee.
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If the IRS determines that you are not withholding enough taxes from your pay check and each year you owe a substantial tax balance, the IRS can send a letter to your employer that requires them to withhold taxes at the highest rate of single with 0 exemptions. This is known as a lock-in letter. This letter sets the withholding rate your employer must use and it cannot be changed by filing a new Form W-4. The rate specified in the lock-in letter can only be changed with IRS approval.
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