Dual status aliens determine their residency status under both the Internal Revenue Code and tax treaties.
Dual Status Alien
If you change status during the current year-
Aliens who make such a change are Dual Status Aliens and must file a special tax return called a Dual Status Return as described in Publication 519, U.S. Tax Guide for Aliens.
Dual Status Alien - First Year Choice
If you are a Nonresident Alien who will become a Resident Alien under the Substantial Presence test in the year following this taxable year, you may elect to be treated as a Dual Status Alien for this taxable year and a Resident Alien for the next taxable year if you meet certain tests. Refer to the First Year Choice area, under Dual-Status Aliens, of Chapter 1 in Publication 519, U.S. Tax Guide for Aliens.
Most Tax Treaties contain an article which defines tax residency for purposes of the Tax Treaty. Tax residency determined under the residency article of a tax treaty may differ from the residency provisions of the Internal Revenue Code.
Dual Status Aliens Married to U.S. Citizens or Resident Aliens
A dual status alien married to a U.S. citizen or to a resident alien may elect to file a joint income tax return with his/her U.S. citizen or resident alien spouse. Refer to Nonresident Spouse Treated as a Resident.