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From 1975 to 1989, the old Region IX (Western Mindanao) was further divided into two sub-regions by "Presidential Decree No. 8233" dated August 21, 1975. Sub-Region IX-A consisted of Basilan, Sulu and Tawi-Tawi with Jolo, Sulu, as the sub-regional center, while Sub-Region IX-B consisted of the provinces of Zamboanga del Norte, Zamboanga del Sur and Zamboanga Sibugay, with the chartered city of Zamboanga City as the sub-regional centre.