A rubber/plastic cap that fits over the lip of a carboy, and has two differently-size nipples to which airlocks and/or small-diameter blow-off tubes can be attached.
The secondary function for a carboy cap is to enable easy syphoning. This can be done by inseting the racking can through the larger whole and pressurising the carboy with the smaller tube to start the syophon. The easiest and most sanitary method is to pressurise with a low pressure blast of CO2.
