Independent personal services: residence-state taxation, but taxable in other state if fixed base or substantial presence. Independent personal services income of a resident individual is taxable only in the State of residence, except that the other Contracting State may tax income attributable to a fixed base there or income derived from activities performed in that State when the individual's presence there meets the substantial presence threshold. 'Professional services' covers independent scientific, literary, artistic, educational or teaching activities and named professions including physicians, lawyers, engineers, architects, surgeons, dentists and accountants.