Identification sign limited in area to one square foot for every six lineal feet of frontage occupied, located only in yards abutting streets, one to each street frontage, erected to a height not less than 10 feet or more than 20 feet above the ground and not projecting into a public street or way. No sign unattached to the wall of a building, however, shall exceed a total of six feet in height, regardless of the lineal feet frontage of the subject premises.