The identification of fastener grades varies between fastener manufacturers. The grade can be identified by circles, slashes, dimples, etc. The SAE has established a sequence of grades from 0 to 8 for steel bolts on the basis of the metal from which the bolt is made and the manner of manufacture. Available grades run from 2 to 8, with 8 being the strongest. Higher-grade numbers almost always mean increased strength (an exception is that some grade 6 bolts are stronger than grade 7). The heads of steel bolts are marked to identify their grade.