A cup square is a domed head bolt with a square anti-spin shoulder.
Cup squares are mainly used to attach wood to wood, for example, the seat and back slats of garden benches to the support structure.
Also used in construction and roofing projects.

Useful to connect metal and wood.

Securing two or more loose parts to one another.

The fastener is designed so that the square shoulder pulls into the wood as the nut on the other end is tightened, thus eliminating the need for a second spanner to prevent the bolt from turning while the nut is tightened.