The Nuttall Encyclopedia
The standard, surmounted by the monogram of Christ, which was borne before the Emperor Constantine after his conversion to Christianity, and in symbol of the vision of the cross in the sky which led to it. It was a lance with a cross-bar at its extremity and a crown on top, and the monogram consisted of the Greek letter for Ch and R.
Wood, James, ed. Entry for 'Lab`arum'. The Nuttall Encyclopedia. https://www.studylight.org/encyclopedias/eng/nut/l/labarum.html. Frederick Warne & Co Ltd. London. 1900.