N.Y.C. Charter § 496: Section 496. Deputies.

The commissioner shall have the power to appoint and, at pleasure, remove deputies, one to be known as first deputy commissioner. During the absence or disability of the commissioner, the first deputy commissioner, or if the first deputy commissioner shall be absent or under disability, the deputy commissioner designated by the commissioner, shall possess all the powers and perform all the duties of the commissioner, except the power of making appointments and transfers.