[SOC] BTU?

[K6KF]

Hello everyone,

This is my first post to the ML. I have a question about CW 
abbreviation.
I have asked a couple of my local hams about this, but it might be good 
to ask here also.

First of all,

1) What's BTU at the end of transmission? I assume it's "Back To U".
2) If so, is it popular on the air?

I am asking because a guy who is a very skillful CW operator told me 
"It NEVER exists". but I indeed heard it several times on the air. He 
added "I have never heard of it in my 30 year CW life. You should get 
on the air first, then listen, you'll see. (I guess it implies "so you 
won't say such a bogus"....)"

I got back on the air last year after having been away from ham radio 
for about 10 years. I am not so active on the air, but still trying to 
improve my CW skills. So I am still kind of  beginner. Now I would like 
to make sure what I heard on the air....

Jun K6KF