In medieval England, France and Scotland, nobles - seemingly of all ranks - flew flags not only displaying their coats-of-arms but also banners with their livery colours and badges. These, depending on the country bore either the cross of St George, St Denis or St Andrew in the 'fly' (the part nearest the flagpole). These were in use from the 14th to I believe the 16th centuries

Is there a definitive literary source for these?

Thanks in anticipation