Part of our Getting On The Air: Modes, Operations, and Making Contacts guide.
Offset and duplex are two of the settings that confuse new hams the most when programming a radio for the first time, mostly because the terminology is used loosely and the concepts get blurred together. They are actually simple once you separate what each one is doing.
Why a repeater needs two frequencies
A repeater listens on one frequency and transmits on another, simultaneously, which is what lets it retransmit your signal in real time instead of recording and replaying it. For it to work, your radio has to transmit on the repeater’s input frequency and receive on its output frequency. Since those two frequencies are different, your radio needs to know how far apart they are and in which direction, and that relationship is the offset.
Standard offsets by band
Most bands have a conventional, widely used offset so coordinators do not have to specify it repeater by repeater. On 2 meters the standard offset is 600 kHz. On 70cm it is typically 5 MHz. These are conventions, not laws of physics, so always check the actual repeater listing, but if a 2 meter repeater does not list an offset explicitly, 600 kHz positive or negative is the safe first guess.
Positive versus negative shift
Positive offset means your radio transmits above the frequency you have dialed in and listens on the frequency itself. Negative offset means the opposite, you transmit below the displayed frequency. Most repeater coordination follows regional band plans for which segments use positive versus negative shift, which is why a repeater directory listing will simply say plus or minus rather than requiring you to calculate it. Program the direction the listing specifies and you never need to think about it again for that particular repeater.
Where duplex fits in
Duplex, in this context, just describes transmitting and receiving on two different frequencies at the same time, which is exactly what a repeater setup requires. Your handheld or mobile radio is not truly full duplex the way the repeater itself is, it cannot transmit and receive simultaneously on its own two frequencies, but the offset setting tells it to switch between the two the instant you release the PTT, which is functionally what matters for using the repeater.
Putting it together
Programming a repeater channel means setting the receive frequency, the offset amount, the shift direction, and usually a PL tone, which I cover separately in What Is a PL Tone / CTCSS Tone?. Get those four things right and the repeater does the rest. I also go deeper on how repeaters function overall in Repeaters: How They Work, but the offset itself is really just telling your radio which two frequencies to juggle.