D.C. Mun. Regs. tit. 31, r. 31-1312
A "licensed taximeter business" sign, bearing the taximeter business license number and meeting the specifications of the Commission, shall, at all times, be hung or mounted on the outside of the premises in such a manner that it is easily visible to the public from outside the building
A taximeter business owner shall not display a "licensed taximeter business" sign if its taximeter business license, or any other necessary license, is expired, suspended or revoked or if it never was licensed.
Each licensed taximeter business shall have affixed to the inside of the glass window thereon, to be clearly legible from the outside, a printed sign bearing its business name, license number, and the Commission's complaint telephone number.
D.C. Mun. Regs. tit. 31, r. 31-1312