D.C. Mun. Regs. tit. 24, § 2344
2344.1 A licensee shall carry any pistol in a manner that it is entirely hidden from view of the public when carried on or about a person, or when in a vehicle in such a way as it is entirely hidden from view of the public.
2344.2 A licensee shall carry any pistol in a holster on their person in a firmly secure manner that is reasonably designed to prevent loss, theft, or accidental discharge of the pistol.
SOURCE: Final Rulemaking published at 62 DCR 9781 (July 17, 2015).