"Channel U"
As mentioned earlier, there were only the three networks and their stations in the area when I was a kid. That did not remain the case. Eventually a fourth station appeared, channel 20, carrying PBS programming.
This was at a time when changing the channel meant getting up and turning the channel tuner, which turned with a bit of a clunk... unless you were fortunate and had a fancy set with (probably ultrasonic) remote control. The channel tuner wasn't a single thing, but a pair: One for the VHF channels (2-13) and one for the UHF channels (14-83). To use the UHF tuner, the VHF tuner was set to "U" which was between 13 and 2 as the dial went around, and then the UHF tuner was used.
When all stations were VHF, the UHF tuner didn't matter much. When channel 20 appeared, there was a use for tuning to UHF. As there was only the one UHF station, there wasn't much point in adjusting it, at least once the fine tuning was set right, so the UHF tuner generally was set and then left pretty much alone. The VHF tuner was used, and I recall hearing people (mostly kids, since I was a kid at the time) refer to the PBS station as "channel U" since the VHF tuner was set to 7, 9, 12, maybe 13, and U.[1] While I understood how that happened, it still annoyed me even then. It was channel 20, not U.
[1] With a good outdoor antenna it was possible to get other channels, though not always very well. These channels were also VHF, most commonly 2, 5, and 11 from Green Bay.