In the history of the Council of Constantinople, it is stated that—
"George, the well beloved of God, a deacon and keeper of the records, having touched the Holy Gospels of God, swore in this manner, 'By these Holy Scriptures, and by the God who by them has spoken,'" &c.
At the Council of Nice it is said that—
"Prayer having been offered up, every one saluted the Holy Gospels, the venerated cross and image of our Lord and Saviour Jesus Christ, and of our Lady the mother of God, and placed his hands upon them in confirmation of what he had said."
From these I infer that the custom of swearing on the Gospels received the sanction of the church at a very early period.
In reply to the question as to other modes of swearing, it may be said briefly, that men swore by anything to which they attached any importance, and generally by that to which they attached most importance.
By the laws of the Alamanni, a wife could claim her Morgen-gabe (or the gift of the morning after the wedding night) by swearing to its amount on her breast; and by the Droits d'Augsbourg, by swearing to it on her two breasts and two tresses.
Nothing was more common than for a man to swear by his beard. This custom is alluded to by one of Shakspeare's fools, who suggests that if a certain knight swore by his honour, and his mistress by her beard, neither of them could be forsworn.
In the canons of the Fourth Council of Orleans, we read—
"Le Roi lui-même, ou le plus renommé des chevaliers présents, ayant découpé le paon, se leva, et mettant la main sur l'oiseau, fit un vœu hardi; Ensuite il passa le plat, et chacun de ceux qui le reçurent fit un vœu semblable."