1. This rule, a duplex one, was not as common as the many simplex models produced by the same company of the same size and dimensions.
2. Elsewhere we have commented on the fact that, despite holding the patent for differential trig scales, some of the Thornton rules had standard type trig scales. This rule has taken that philosophy a stage further - it has both types.
3. In addition to the normal Pythagorean scale, Ö(1-x²) , designated Ps, this one has a scale of Ö(1+x²) designated Pt. This is designed for calculating the sum of two squares, since, in a right angled triangle:
   Hypotenuse = Ö(a²+b²)   = aÖ(1+(b/a)²)