Button head bolts are similar to standard pan heads but have a curved top.
Fillister head bolts have a thick cylindrical head with a deep drive slot for higher levels of torque. The head is smaller than a round head.
Flange hex head bolts have an extra bearing surface between the top of the bolt head and the body that provides an additional gripping strength. This flange area resembles a flat washer, with its semi-flat disc shape.
Flat head bolts are designed to fit flush to the surface when used with countersunk holes.
Hex head bolts are hexagonal shaped with a flat bearing surface.
Large diameter round head (elevator) bolts are thin and flat, giving the bolt low head clearance.
Oval head bolts are tapered on the bottom like flat heads to fit flush in countersunk holes, but they have an oval top.
Round head bolts have a dome shape. They are considered an older style and are commonly replaced by other heads.
Socket cap bolts, also known as Allen head bolts, have a recessed drive, and are commonly used when there is not enough clearance for a conventional wrench or socket. They are fastened with an Allen wrench, which is also called a hex wrench or hex key.
Square head bolts have a square shaped, prominent head that is easy to access with a wrench or by hand, when more torque is needed.
Truss head bolts have an extra-wide lower-profile dome with a slightly rounded top.