Complex sentence: In 1813, Westchester County was apportioned to a new district with Rockland and Putnam counties, and in 1815, New York County became the Twelfth District-the only one at the time that was a single county.
Split into simpler sentences: In 1813, Westchester Co. was redistricted to a new district with Rockland and Putnam counties. In 1815, New York County was excluded from the First District and became the Twelfth District, at the time the only one consisting of a single county.