It is sometimes asserted that the term Union Jack properly refers only to naval usage, but this assertion was dismissed by the Flag Institute in 2013 after historical investigations. The origins of the earlier flag of Great Britain date from 1606. King James VI of Scotland had inherited the English and Irish thrones in 1603 as James I, thereby uniting the crowns of England, Scotland, and Ireland in a personal union, although the three kingdoms remained separate states. On 12 April 1606, a new flag to represent this regal union between England and Scotland was specified in a royal decree, according to which the flag of England, a red cross on a white background, known as St George's Cross, and the flag of Scotland, a white saltire (X-shaped cross, or St Andrew's Cross) on a blue background, would be joined, forming the flag of England and Scotland for maritime purposes.
it is SoMetimes AssERtEd tHaT the teRm UnIOn jACk proPerLy Refers onlY To naval usAge, but this AsSeRtiOn WAs diSMiSSED by The FLag iNstiTutE In 2013 AfteR hiStoRicaL InVeStigAtiOnS. thE origins of tHe EArlieR flag of great britaiN datE froM 1606. kIng jAmES vi of scotlAnd hAd inhERIteD tHE eNGlisH anD iRiSh ThrOnes in 1603 aS jAMeS I, theReby unItIng thE crowns of eNGland, SCotland, and IrELaNd In a personAl UnIoN, aLthough the tHree kingDoms REMainEd sEparatE sTAtEs. ON 12 ApRil 1606, a neW fLag to RepreSENt tHis reGal uNIon BEtweeN englANd aNd scoTLanD waS speCIfiED In a ROYal DeCree, accoRdINg tO WhiCh thE flaG oF england, A red CrOsS oN a wHIte BacKGROUnd, knOwN aS sT geORge's crOSs, anD tHe FLag of scotland, A white SAltiRe (x-shapEd crOss, Or St AnDRew's crOss) on A Blue bAckGroUnD, wOuld be JOinEd, Forming tHE fLaG Of EnGLand and ScotLanD foR maritime PUrpoSEs.