Senatorial districts of the Philippines

The enactment of the Philippine Autonomy Act (popularly known as "Jones Law") in August 1916 by the United States Congress provided for the creation of a bicameral legislature consisting of a lower chamber (House of Representatives) and an upper chamber (Senate).

The system of government of the Philippines in its early years of transition to democratic self-government was deliberately structured to emulate the American model.

The first eleven districts were composed of established provinces, while the twelfth was composed of the provinces of the Luzon interior and much of Mindanao – both of which were never fully administered by the old Spanish colonial government and designated by American authorities as "non-Christian" areas.

The setup lasted until the establishment of the Commonwealth of the Philippines in 1935, when the bicameral legislature was abolished, as the 1935 Constitution provided only for a unicameral National Assembly.

However, when the Constitution was amended in 1940 to re-establish a bicameral Congress, members of the Senate had to be voted at-large, thereby effectively abolishing the district system.