Independent personal services: taxable in resident state unless fixed base or substantial presence permits source taxation. Income from independent personal services is taxable only in the taxpayer's residence state except where the taxpayer has a fixed base in the other Contracting State, in which case that State may tax income attributable to that fixed base, or where the taxpayer's presence in the other State meets the treaty's substantial presence threshold, in which case that State may tax income from activities performed there.