Washington (CNN) - Following Monday's deadly shooting at the Washington Navy Yard, President Barack Obama ordered all flags on U.S. military installations and federal government facilities to be lowered to half-staff out of respect.
But since the attack occurred at a U.S. naval facility, those flags, as well as flags flown at Marine Corp facilities and on naval vessels, are actually lowered to half-mast (consistent with masts on ships).
The action is the same in both cases - first raising the flag to the top of the flag pole, then lowering it half way - but the terminology is different on land vs. on sea (and at Navy/Marine facilities).
Also, it should be noted that Vice President Joe Biden lives on a naval facility - the Naval Observatory in Washington, D.C. - and the presidential retreat at Camp David is formally a naval facility, so those flags would also be lowered to half-mast.
According to the president's proclamation, flags will "be lowered to half-staff immediately and remain at half-staff until sunset on Friday, September 20. "