A sign appeared to the Roman Emperor Constantine as a light in the noon-day sky. At the same time, he heard the words, In Hoc Signo Vinces (In this sign you shall conquer). He took this as a sign from God and decided the shape drawn by the light should be an emblem for his military. This emblem became known as Labarum (etymology unclear - possibly a Gaulish/Celtic word. See Lauburu Cross).
The shape he saw was of a flaming cross - "a long spear, overlaid with gold". It had a cross bar and "on the top of the whole was fixed a wreath of gold and precious stones, and within this the symbol of the Saviour's name, two letters indicating the name of Christ by means of the initial letters, the letter 'X' intersection 'P' at the centre." (recorded in the 3rd century by Eusebius Pamphili, BishEusebius' description of the labarum
Constantine's labarum, from an antique silver medal
"A Description of the Standard of the Cross, which the Romans now call the Labarum." "Now it was made in the following manner. A long spear, overlaid with gold, formed the figure of the cross by means of a transverse bar laid over it. On the top of the whole was fixed a wreath of gold and precious stones; and within this, the symbol of the Saviour’s name, two letters indicating the name of Christ by means of its initial characters, the letter P being intersected by X in its centre: and these letters the emperor was in the habit of wearing on his helmet at a later period. From the cross-bar of the spear was suspended a cloth, a royal piece, covered with a profuse embroidery of most brilliant precious stones; and which, being also richly interlaced with gold, presented an indescribable degree of beauty to the beholder. This banner was of a square form, and the upright staff, whose lower section was of great length, of the pious emperor and his children on its upper part, beneath the trophy of the cross, and immediately above the embroidered banner."
"The emperor constantly made use of this sign of salvation as a safeguard against every adverse and hostile power, and commanded that others similar to it should be carried at the head of all his armies."