This letter informs the IRS that you have underpaid your taxes and therefore owe the government money.
1234, Main Street
Boston, MA 02123
05 April, 2005
<Recipient Address Goes Here>
For the tax year <year>, <I/we> have discovered that <I/we> underpaid <my/our> <federal/state> income tax. <I/we> paid <$amount>, while upon further calculation <I/we> should have paid <$amount>. <I/we> believe this underpayment means <I/we> owe the <state/federal government> a balance of <$amount>, plus an interest and penalties.
Please contact <me/us> as soon as possible to let <me/us> know about any interest or penalties that may be applicable. Thank you for your time.
encl: <List of enclosed items goes here>