For individual returns (Forms 1040), the IRS usually applies the six-year rule, unless it suspects significant potential taxes owed in the years prior to the past six years. Most of the time, when the IRS deviates from the six-year rule, the delinquent-return investigation is usually assigned to a local IRS collection office (to a revenue officer). IRS Policy Statement (5-133) doesn’t apply to business returns (employment tax returns Forms 940, 941, 944; income tax return Forms 1065, 1120, 1120S, and many other types of returns).