the basic reason is that ASR33 teletype machines have a backarrow character, whose ASCII code is the one more commonly associated with underscore. When I read

the basic reason is that ASR33 teletype machines have a backarrow character, whose ASCII code is the one more commonly associated with underscore. When I read this I said "huh?" because I used ASR33's quite a bit in my youth and I didn't remember that. But, sure enough, the 1963 version of ASCII had the backarrow. It was replaced by underscore in the 1965 version. The backarrow would have been current when Iverson first developed APL. https://en.wikipedia.org/wiki/ASCII#Printable_characters https://en.wikipedia.org/wiki/ASCII#Printable_characters