BASH shell only - brace expansion

You might have used brace expansion if you have tried this very popular way of backing up files - 

cp filename{,.bk}

The command above will copy filename to filename.bk. It saves you from writing the filename again. 

This neat stuff is not compatible with korn shell.

Some other examples to reinforce how this works.

echo John{Doe,Smith}
  JohnDoe JohnSmith
echo {John,Jane}{Doe,Smith}
  JohnDoe JohnSmith JaneDoe JaneSmith
  

Note the lack of space after comma.