The Geus is a Jack hoisted by Royal Netherlands Naval ships on Sundays, national days, and in foreign harbors or when foreign ships are present in its own harbor. The Jack originated from the flag use by the "Guezen", rebels against the Spanish during the 80 year war between Holland and Spain in the second half of the 16th century which led to the independence of Holland from Spain.

(Information on the Geus was provided by Captain Frederik Laks, Royal Netherlands Navy, retired. He served as navigator on the HNLMS Walrus (ex USS Icefish, SS367) in 1970 and as Commanding Officer of the HNLMS Zeehound from 1977 to 1980.)