Independent personal services: source taxation allowed where a fixed base exists or stay meets treaty threshold. Income from independent personal services by a resident is taxable only in the State of residence except where the individual has a fixed base in the other State-then only income attributable to that fixed base may be taxed there-or where the individual's stay in the other State meets the treaty duration threshold-then only income from activities performed in that State may be taxed there. 'Professional services' encompasses a broad list of independent occupations including specified professions.