Independent personal services: source-state taxation permitted where a fixed base exists or prolonged presence triggers taxation. Income from independent personal services of a resident is taxable only in the state of residence except where the individual has a fixed base in the other State-then income attributable to that fixed base may be taxed there-or where the individual's aggregate presence meets a presence threshold over 12 months-then income from activities performed in that State may be taxed there; 'professional services' includes specified independent occupations and activities.