Posted on 21st Jun 2011 @ 3:58 PM

While I've primarily heard "flying a flag at half-staff," I wasn't certain of the reasons for using either term with the American flag. So we put a little research time into this. One of the first things I found was a good article dealing with this exact question from a Canadian perspective.

The US Flag Code TITLE 4 > CHAPTER 1 > Sec. 7. "Position and manner of display" consistently uses the phrase "half-staff" throughout subsection (m) including "(m)(1) the term ''half-staff'' means the position of the flag when it is one-half the distance between the top and bottom of the staff;"

A Google search of the whitehouse.gov website shows consistent use of the phrase "Half-staff" and no use of "half-mast"
There is one example in a recent White House press release related to an Executive Order on when to fly the flag at half-staff.

On the other hand, both dictionary.com and Merriam-Webster online defer any definition of "half-staff" & instead list "see half-mast." Definitions for half-mast from those 2 sources:
" The position about halfway up a mast or pole at which a flag is flown as a symbol of mourning for the dead or as a signal of distress. Also called half-staff."
" a point some distance but not necessarily halfway down below the top of a mast or staff or the peak of a gaff" Neither definition makes any reference to the use of half-mast as a nautical term.

The North American Vexillological Association primarily uses "half-staff" but does not address your issue when answering a question that used the term "half-mast."

In summary, while we have been unable to find a source to back up the CBC article's comment "...halfstaff is acknowledged as the official expression in the United States," anecdotal evidence for American use of "half-staff" is overwhelming when used in the context of lowering old Glory in respect for the dead. That being said, I would not be personally offended by anyone using "halfmast" in this context as it does not appear to be an issue of disrespect.