 The "triangle slide" is the method NAMeS uses to estimate the largest triangle that can be inscribed in a fillet weld.
The AWS Standard Terms and Definitions often refer to the largest triangle that can be inscribed in the weld bead. The calculation to determine this is intensive, so this triangle is approximated by the Fillet model.
First a base line is established. The line could just be an arbitrary line at a fixed angle like 45 degrees. Or this base line could be the line passing through the outermost points of the weld toes. Or the base line could be the line passing through inner toe points where penetration begins.
A surface is defined that the line must be below. This surface could be just the top surface of the weld bead (from the outer toe points where the weld wets the surface of the plate). Or the surface could be extended to include the points where the weld first penetrates into the plates (the inner toes) 
