Taxation of independent personal services: residency principle with fixed-base and presence-threshold exceptions for source-state taxation. Income from professional or similar independent activities received by a resident individual is taxable only in the State of residence, except where the individual has a fixed base in the other State, in which case the other State may tax income attributable to that fixed base, or where the individual's presence in the other State meets the applicable presence threshold within a relevant twelve-month period, in which case the other State may tax income derived from activities performed there. 'Professional services' expressly includes specified independent scientific, artistic, educational and professional practices.