In order to listen and transmit at the same time, repeaters use two different frequencies. On the 2 metre ham band these frequencies are 600 kilocycles/second (or 600 kilohertz) apart. As a general rule, if the output frequency (transmit) of the repeater is below 147 Mhz then the input frequency (listening) is 600 kilohertz lower. This is referred to as a negative offset. If the output is above 147 Mhz then the input is 600 kilohertz above. This is referred to as a positive offset.