It was created in June 1999 as a split from area code 616. 231 is essentially the northern half of the old 616 area code. 616 previously covered the entire western half of the Lower Peninsula, extending west from the north-south line aligned with the Mackinac Bridge. Generally, 231 includes the western half of the territory generally defined as Northern Michigan. Muskegon is usually defined as being part of West Michigan, but now uses 231 rather than 616.