A broad command pennant of the U.S. Navy.  These are flown flown in lieu of the commission pennant by commanders of squadrons of ships who are not flag officers (i.e. admirals, who have their own flag designs).  The number of the specific unit is placed on it.  This type of pennant has a long history; see here for more details.

The pennant shown is that of Destroyer Squadron 23, commanded by then-Captain Arleigh Burke during World War II in the Pacific.
Own work
Carl Lindberg