Most countries have a series of 'tests' to determine tax residency. I can't remember all the details, but a lot of it comes down to substantial ties. i.e. spou

Most countries have a series of 'tests' to determine tax residency. I can't remember all the details, but a lot of it comes down to substantial ties. i.e. spouse, family, primary residence, secondary residences, number of days spent in the country, number of years away from country, etc. Countries solve the issue of 'competing' for an individuals tax residency through tax treaties which lay out the method in which the tests are applied and who would 'win' in a tie-break situation. This avoids the issue of double taxation but means you'll be subject to tax somewhere in any one year. That's likely where the 180 days figure comes from as it's one of the most broad and substantive tests to determine tax residency, but it's not the only metric used. The actual details are more nuanced. For example, if you give up tax residency in one country but then spend 179 days there each year to visit your spouse and kids – they'll probably claim you're tax resident.