My father, a violinist, had a copy, and my brother, a cellist, no doubt looked into it during his musical education. I was always puzzled by where all of these rules came from and what their purpose was, but I could never get an explanation.
[Raak] The "rules" spring from what is considered consonant and dissonant, views of which have changed throughout musical history. Hence you can end up with a fugue like that in Walton's 1st Symphony, which would have Prout screaming in his grave. All good clean fun