Independent personal services: source-state taxation permitted where a fixed base exists or presence exceeds prescribed threshold. Independent personal services income of a resident is taxable only in the State of residence except where the individual has a fixed base in the other State-allowing taxation of income attributable to that base-or where the individual's presence in the other State meets a duration threshold, permitting taxation of income derived from activities performed there. The Article specifically includes independent professional activities such as scientific, literary, artistic, educational work and independent medical, legal, engineering, architectural and accounting practices.