Taxation of independent personal services: residence taxation plus potential source taxation where presence or a fixed base exists. Income from independent personal services paid to a resident is taxable in the resident State but may also be taxed in the State where services are performed if the provider meets a qualifying period of presence there or has a fixed base regularly available; only income attributable to that fixed base is taxable by the source State. 'Professional services' includes independent scientific, literary, artistic, educational or teaching activities and the independent activities of physicians, surgeons, lawyers, engineers, architects, dentists and accountants.