Which mentions a pre-defined ‘square’ used to calculate the angles for a certain lines (like on the page given the ‘seat angle’ for a pair of trousers).

Not a priority, but does this have an equivalent tool in Seamly2D? Alternatively I can possibly examine his sample given prior to this in the notes and get the relevant angle by trig functions.

I’m aware of that, but in the diagram in the book, that’s not technically what the ‘square’ is doing in this instance. It’s being placed to get what it terms the ‘seat-angle’, for a line that is NOT perpendicular to the line measured as the “seat” in the pattern ( I am perhaps wrongly thinking this translates as the hip/highip line in modern terms). I can find the angle with some trig functions I think.

It may be worth adding a comment in the documentation about older pattern systems approaches, but as Vincent’s system is at least 100 years old, It’s hardly a priority, and so more recent systems should be fully documented first.