Treaty tie-breaker

When domestic law makes you a resident of both countries at once, the residence article of the treaty (usually Article 4) resolves it through an ordered cascade: permanent home available to you, then centre of vital interests, then habitual abode, then nationality, and finally mutual agreement between the two tax authorities. Each step is only reached if the previous one fails to break the tie. The result matters much less for US citizens than people expect: the saving clause preserves the US right to tax its citizens on worldwide income, so tie-breaking to the other country does not switch off the US return. For a green-card holder the consequence is real and often unwelcome — being treated as a resident of the other country under a tie-breaker makes you a nonresident for US income-tax purposes under §7701(b)(6), which requires Form 8833 disclosure and can start the clock on expatriation consequences.