A saltire, also called Saint Andrew's Cross or the crux decussata, is a heraldic symbol in the form of a diagonal cross. The word comes from the Middle French sautoir, Medieval Latin saltatoria.
Coin of Theodosius II (425–429), showing the emperor with globus cruciger and with the same vexillum
Saint Andrew martyred on a decussate cross (miniature from an East Anglian missal, c. 1320)
Quarterly 1st & 4th: Barry of six [seven] vair and gules; 2nd & 3rd: Gules, a saltire vair (Henry Beaumont of Devon, d.1591)
Papal coat of arms for Pope Innocent VIII with the Keys of Peter saltirewise (Wernigerode Armorial, c. 1490)
A jack is a flag flown from a short jackstaff at the bow of a vessel, while the ensign is worn on the stern. Jacks on bowsprits or foremasts appeared in the 17th century. A country may have different
Royal Netherlands Navy jack on HNLMS Walrus (1985), 2008
The US naval jack (2002–2019) being hoisted on USS Kitty Hawk's jackstaff in December 2001
Naval jack of the Irish Naval Service
Russian Navy submarine Sankt Peterburg displaying the Russian naval jack at the bow and the naval ensign at the stern