By FCC law. How many kHz away for a QSO are you allowed to start another QSO?
I believe that 3 kHz is the FCC rule. Another Ham says that it's two.
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The truth is, there is no such regulation, except for this one, from Part 97. Bottom line is, the law is to not cause harmful interference. Good practice is to be at least 2 kHz to prevent bleed through, although 3 kHz is better. SSB emissions take 2.8 kHz. Of course, if you are using a digital mode, you can be quite a bit closer, and cause no interference at all.
§97.311 SS emission types.
(a) SS emission transmissions by an amateur station are authorized only for communications between points within areas where the amateur service is regulated by the FCC and between an area where the amateur service is regulated by the FCC and an amateur station in another country that permits such communications. SS emission transmissions must not be used for the purpose of obscuring the meaning of any communication.
(b) A station transmitting SS emissions must not cause harmful interference to stations employing other authorized emissions, and must accept all interference caused by stations employing other authorized emissions.
(c) When deemed necessary by a District Director to assure compliance with this part, a station licensee must:
(1) Cease SS emission transmissions;
(2) Restrict SS emission transmissions to the extent instructed; and
(3) Maintain a record, convertible to the original information (voice, text, image, etc.) of all spread spectrum communications transmitted.
As mentioned, the law is not to cause harmful interference.
You can be any frequency spacing from, or no spacing from another station if you don't cause interference.
For example, if two stations some distance from you are hearing each other, you could use a very close frequency or even the same frequency to talk to another station close to you and as long as the other stations didn't hear it, no problem.
They might be interfering with you, but that is also no problem if you are willing to accept it. In the afternoon on 75 meters and in a large country such as the U.S., for example there could be multiple groups of stations on each frequency with minimal or no interference. Even if they occasionally hear each other, no one is illegal, provided they do not prevent other users from carrying on desired communication.
When the "skip" lengthens out, harmful interference may occur and it can lead to arguments as to who was on the channel first and thus should be protected from the newer station's use of the frequency.
The only answer to such a situation is mutual respect and courtesy leading to cooperation in frequency selection, scheduling, power levels, etc.
Hams are generally left to work this out among themselves and it works pretty well.
Already good answers but there is one more thing. Once you have chosen a frequency, you should ask if it is busy. If using voice mode, it is OK to say something like "Is this frequency busy?" and then pause for some time to listen for any answers. Other modes justify other codes. For example, CW usage would be to send QRL? which is taken to mean (in our current interpretations) "is this frequency busy?". And, then of course listen for an answer. In CW, a common answer is just the letter C if someone is already using that frequency. Sort of meaning "Yes, the frequency is busy".
So, good practice is: