Imagine a company headquartered in the UK builds a factory in Vietnam. When the factory turns a profit, who gets to tax it? The UK, where the company is legally resident, argues it has the right to tax its company's worldwide income. This is the principle of residence-based taxation. Meanwhile, Vietnam, where the profit was actually generated, argues it has the right to tax economic activity within its borders. This is source-based taxation.

Without a clear agreement, the company could be taxed twice on the same income, once by the UK and again by Vietnam. This issue of double taxation would severely discourage international trade and investment. To solve this, countries enter into Bilateral Tax Treaties (BTTs), which act as a rulebook to divide the taxing rights between the residence state and the source state.

Countries don't start from scratch when negotiating a treaty. They typically use one of two main templates, or models. The choice of model reveals a lot about the economic relationship between the two nations.

The first is the OECD Model Tax Convention . Developed by the Organisation for Economic Co-operation and Development, which consists mainly of developed, capital-exporting countries, this model heavily favours residence-based taxation. It generally gives the exclusive or primary right to tax income to the country where the company is headquartered. The logic is that the capital, expertise, and intellectual property originated there.

The alternative is the UN Model Double Taxation Convention. Developed with the needs of developing, capital-importing countries in mind, this model grants broader taxing rights to the source country. It allows the country where the economic activity occurs to tax more types of income, recognising that this is where the value is created and where public services are used to support the business.

Most treaties are a hybrid, blending elements of both models to reflect the specific balance of trade and investment between the two signatory countries.

Once the treaty decides which country gets to tax what, it must also provide a mechanism to prevent double taxation. There are two primary methods: exemption and credit.

The exemption method is the simpler of the two. The residence country simply agrees not to tax the income that has already been taxed in the source country. For example, if a UK company earns £100 in Vietnam and pays £10 in Vietnamese tax, the UK simply ignores that £100 of profit when calculating the company's UK tax bill.

The credit method is more common and complex. The residence country includes the foreign income in its tax calculation but provides a credit for the taxes paid to the source country. Let's say the UK corporate tax rate is 25%. On the £100 profit from Vietnam, the UK tax would be £25. Since the company already paid £10 in tax to Vietnam, the UK gives a credit for that amount. The company then only pays a "top-up" tax of £15 (£25 - £10) to the UK government. If the Vietnamese tax had been £30, the UK credit would be limited to £25, and no further UK tax would be due.

A key tool used by source countries is withholding tax. This is a tax deducted at the source on payments like dividends, interest, and royalties sent to a foreign entity. For instance, if a Vietnamese company pays royalties to its UK parent, the Vietnamese government might withhold 15% of that payment as tax. Tax treaties are crucial here, as they almost always negotiate to reduce these withholding tax rates, often to 5% or 10%, to encourage investment.

Some treaties, particularly between developed and developing nations, include a [{] credit. Imagine Vietnam offers a 10-year tax holiday to a new factory to encourage investment. Normally, under the credit method, the UK company would get no tax credit (since no tax was paid) and would just pay the full 25% tax in the UK. This nullifies Vietnam's incentive. A tax sparing clause means the UK gives a credit for the tax that Vietnam would have collected, preserving the benefit of the tax holiday for the investing company.

Finally, the credit method can be extended through indirect tax credits. A simple credit only applies to taxes directly paid, like withholding tax on a dividend. An indirect credit allows the UK parent company to also claim a credit for the underlying corporate income tax paid by its Vietnamese subsidiary on the profits from which the dividend was paid. This provides a more complete form of relief from double taxation, acknowledging that the subsidiary's profit was already taxed once before being distributed.

Tax treaties are the essential plumbing of the global economy. They balance the competing interests of nations and provide the certainty needed for businesses to operate across borders.