A petar was a medieval small bomb used to blow up gates and walls when breaching fortifications. In a typical implementation, it was commonly either a conical or rectangular metal object containing 5 or 6 pounds of gun powder, activated with a slow match used as a fuse. It was often placed either inside tunnels under walls, or directly upon gates. When placed inside a tunnel under a wall and exploded, large amounts of air would often be released from the tunnel, as the tunnel collapsed. By securing the device firmly to the gate, the shape of the device allows the concussive pressure of the blast to be applied entirely towards the destruction of the gate. Depending on design, a petard could be secured by propping it against the gate using beams as illustrated, or nailing it in place by way of a wooden board fixed to the end of the petard in advance.