很多业余无线电爱好者在使用CW模式通联结束时，都会加上2个点做为结束语，其实这种操作还是一个火车拉来的礼仪。
In the past, each train would emit two short blasts of its horn as it departed from the station to alert everyone that it was leaving. Some experienced former switchboard operators had adopted this practice in their own communications, using the blasts at the end of each call to say "goodbye."
Due to its simplicity and convenience, this method is particularly useful at the end of a communication session, without reducing efficiency. As a result, it has been widely adopted by amateur radio enthusiasts in their CW mode communications.
This practice has persisted in amateur radio circles to this day.
Below is a real historical account of the experiences of amateur radio enthusiasts in the early 1960s:
ERIC, K1NUN writes: "When I was first licensed in the early 1960s, we would end our CW conversations with 'di dididi di (Shave and a haircut)' to which the other station would reply with 'di di (two bits)'. Then station 2 sent 'di dididi di' and the first would reply with 'di di'. Sometimes, and particularly among us young teenagers, this exchange might continue for several minutes. Over time, this back-and-forth terminal handshake shortened just to 'di di'. Also, about that time, sometimes instead of calling CQ, one would just send 'di dididi di' over and over on a frequency (remember, we were crystal-controlled in the novice bands) while tuning the receiver, listening for anyone who sent a synchronized 'di di'. After enough back-and-forth to ensure proper synchronicity, one of the stations would send 'QRZ?' (with the question mark so as to really ask who is calling me). Thus, 'di dit' also began a QSO in some cases. Fortunately, this custom died within a very few years as the rules changed and more of us could use transceivers that put us right on frequency and we did not have to tune around and search for responders. And that's my story, and I lived it."
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