A similar functionality can be provided for masks without an integral bailout block, by mounting an external bailout block at a convenient place on the diver's harness. Surface supply and bailout supply hoses are connected to this block, which works exactly the same way as the integral block. The same method can be used for open circuit scuba diving, but this only allows for a single gas switch. Gas manifolds with more than two gas supply options are technically possible, but allow for a wider range of operator error and are therefore considered high risk, as the consequences of switching to a gas unsuitable for the current depth may be fatal. This system does not allow for supply of bailout gas to another diver. This system is used for most full face masks, as they have no other option. Surface supplied divers use the pneumofathometer hose to supply gas to another diver.