Two-way communication at sea is still commonly conducted using radios on medium frequency (MF), high frequency (HF), or very high frequency (VHF). VHF is most commonly used by private boat owners because it is cheaper and more compact than the other kinds – but its range is much less. In what is called simplex working both parties speak over the same channel, and you cannot receive while your ‘transmit’ button is pressed. Hence each party must say ‘over’ to signal to the other that he has fInished speaking and is ready to listen. Duplex operation works on two channels, and permits ordinary conversation without need for the ‘over’ drill. Single Side band (SSB) is capable of transmitting across oceans.