Some writers have derived the word jack from Jacques for James I., because he was the monarch who united the flags of England and Scotland; but this is held to be incorrect. The old heraldic name for a surcoat was "jacque," hence obviously our word "jacket," which recalls the German jacke for coat, and therefore undoubtedly "jaque" survives in the "Union Jack," which is intended to represent the national arms, and thus certainly fulfils the purposes of a coat of arms.
The Union Jack first came into use after James I.'s accession, when England and Scotland became united. Till then, the English flag bore St. George's Cross, a rectangular red cross on a white field, whilst the Scotch flag showed the white diagonal cross of St. Andrew on a blue ground.
The union of the two flags was effected by retaining the blue field of St. Andrew's Cross, whilst the red field of the English flag was represented by adding a narrow border of that colour to the limbs of St. George's Cross. The heraldic term for this addition is "fimbrication"—literally bordering. This combined flag remained in use till 1801, when, Ireland having joined the Union, it became necessary to incorporate the cross of St. Patrick into the national banner. But, lest it should be thought that either of the diagonal crosses took precedence of the other, care was taken that the white and red borders of each should be alternately uppermost.
The Lord Lieutenant of Ireland flies the Union Jack with the harp of Ireland on an escutcheon charged upon it. So also does the Governor of India, but in this case the Union Jack bears the Star of India in the centre, charged with a rose, and surmounted by an imperial crown.
We have three flags constantly in use nowadays, which are always spoken of as ensigns. These are:
First, the Red Ensign, a plain red flag, bearing a Union Jack in a canton on the dexter side. This is know as the "Ensign of England," and when displayed at sea distinguishes all vessels not belonging to the Royal Navy.
Second, the White or St. George's Ensign; the original banner of St. George with a "jack" cantoned in the first quarter. This is the ensign of the Royal Navy.
Third, the Blue Ensign, a plain blue field with the Union Jack cantoned in the dexter side. This is the ensign of the Naval Reserve.
The Admiralty flag, displaying a yellow anchor and cable set fesswise on a red field, may be grouped with the three ensigns.