New York City mayor John Francis Hylan's original plans for the Independent Subway System (IND), proposed in 1922, included building over of new lines and taking over nearly of existing lines. The lines were designed to compete with the existing underground, surface, and elevated lines operated by the Interborough Rapid Transit Company (IRT) and Brooklyn–Manhattan Transit Corporation (BMT). The IND Sixth Avenue Line was designed to replace the elevated IRT Sixth Avenue Line. However, since the Sixth Avenue corridor was such an important subway link, the elevated remained open while construction on the Sixth Avenue subway proceeded.