Independent personal services: residence-based taxation except where a fixed base or prolonged presence permits source-state taxation. Income from independent personal services by a resident is taxable only in the resident State, except where the resident has a fixed base regularly available in the other State or where the resident's stay in the other State meets the treaty's prolonged-presence threshold; in those cases only income attributable to that fixed base or derived from activities performed in the other State may be taxed there.