[TheMiller]

I believe that this convention is borrowed from Common Lisp. I always thought of it more as a way to highlight importance. Three semicolons for block comments outside of top-level constructs, which usually explain the purpose or usage of the following expression (usually a function definition). Two for comments inline with function body code. And one for trailing comments on a line, where space is limited and the comment is least important to understand code structure.

The conventions are even mentioned in the Common Lisp specifications: http://www.lispworks.com/documentation/HyperSpec/Body/02_ddb...

[lispm]

The use of one or more semicolons is older than Common Lisp. Maclisp had it in the 1970s, but earlier with different semantics - it had semantics for formatting code. Then it was changed a bit with the use of Emacs.