On Fri, Aug 26, 2022 at 01:15 AM, Jim Brown wrote:

Most of us old-timers spend a LOT more time listening than CQing, so except
for skip, we often DO know whether a frequency is in use because we listen
before we transmit.

Well, we THINK we know. But, for example, two QRP stations may be holding a QSO, at least one of which might be able to hear my higher-powered signal when I can't hear him at all. We may well figure that, for all practical purposes, we know whether a frequency is in use; what I'm talking about is knowing ABSOLUTELY, which my words tended to imply. I imagine most of us have had the experience of shifting propagation suddenly bringing another in-progress QSO to our ears right on top of the QSO we're holding. We thought we were the only stations on that frequency, put changing propagation revealed otherwise. So I only meant to say that I didn't intend my words in that kind of absolute sense, not that we could never have any reasonable confidence at all that a frequency is in fact clear.