It's a holdover from Merry Old England, actually. Sheriff (shire-reeve) is specifically a county (shire) officer (reeve). So we're really a perpetual Medieval Times LARP.

The sheriff is at least elected in most places. Police departments are usually appointed by the council. Most of the time it doesn't change anything, but occasionally a comrade slips into the position after the 95 year old incumbent croaks and people forget that you vote for a sheriff.

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