Ahhh that great debate:

First off we are really not even sure it is called a Barber's Notch see here

http://straightrazorpalace.com/razor...tml#post578486

Second they were around before the Civil War... although I had heard that reason before too...

Some very, very early razors had a clipped point like this one...

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Perhaps it evolved from that, so far there is no definitive answer to the question, although many have put forth tons of good guesses, maybe someday we will find the real answer...