While trying to learn my CW ready for TX and RX, I'm finding there are a number of CW prosigns that I've not really come across before. Ones such as BT, AR and VA. In my search for these I've come across this site:
Also, in CW it's interested me that for signal reports they use the whole RST (not just RS like for voice). I've always wondered what is trying to be achieved with the Tone element of RST, so for that I've come across this:
From that I can see why most the time a T of 9 is sent. However, it is useful to see there are non-numeric options for the T value such as X, C and K.