A duplex check ("cheque" in some ancient climes) is one that makes a copy of itself as you write it.  

Under each check in the checkbook is a sheet of NCR paper, so that as you write, what you write is reproduced on the NCR paper.   The NCR paper is formatted as the check, but does not bear any account numbers.

A proper fountain pen does not exert enough pressure to create the copy.

Aside:  I use electronic payments sparingly, not because I'm agin' 'em, but because I'm afraid I'd lose track.  I spent lots of years figuring out ways not to overdraw my checking account and I don't want to change now.