The current U.S. Navy commission pennant, flown day and night aboard U.S. Navy ships under commission.  It can be displaced only by admiral or other personal command flags, per Navy regulations. 

Today, these are relatively small (the larger version is 2.5 inches tall and six feet long, depicted here).  In the days of sail, they could be up to 100 feet long, and even in 1912, the largest version was 70 feet long.  These larger versions had 13 stars instead of 7.  The current maximum size dates from 1933.  For more information, see here, here, and Navy document NTP 13(B), particularly section 1705.
Own work
Carl Lindberg