I was just rereading this thread tonight to see if I finally understand....

So to simplify .... could it be said that square rule and mill rule are essentially the same the difference being...

In square rule you run a chalk line first (preferably on an edge of a mortise or tennon)...and then you work from the chalk line instead of a milled edge

If this is so... in mill rule when we score a line we have a nice solid edge to hold the square... how do you go about scoring a line in square rule.... besides carefully??

Carlos