The 1973 New Section Operation reorganized the Twelve Regions into six: East Central, North Central, Northeast, Southeast, South Central, and Western. Regions were divided into areas, which were then divided into sections. It is at the sectional level that lodges participate in annual conclaves.
Almost twenty years later in 1992, the Six Regions were consolidated into the Four Regions.
Works Cited: "New Section Operation," Order of the Arrow Bulletin 4 (1972): 4. Print.
North Central Region lodge and section boundaries from 1973-92 (pdf)