In reading any old pattern tailoring book, I came across this :
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.
You will be able to create perfect 90 degree angles using the “point along perpendicular” tool 
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.
Thanks for the suggestion BTW 
An Update, following the measures given in Vincent’s drafting (Vincent's systems of cutting all kinds of tailor-made garments : part 1 : dealing with the cutting of various styles of trousers, breeches, knickers, pantaloons, leggings and gaiters, and waistcoats : Vincent, W. D. F (William D. F.) 1860-1925 : Free Download, Borrow, and Streaming : Internet Archive) , I’ve determined that what the square is providing is most likely an angle of somewhere between 68 and 72 degrees for the seat angle… (most likely 70 degrees, but I couldn’t confirm this, with the precision of measures given.). Currently Seamly2D doesn’t have a known “measurement” for this, but I could set a custom one.
you can also use the point at distance and angle function if you happen to know the angle that you want
I will also note (for reference) an observation on the pattern proceeding the page mentioning the Vincent “Square”.
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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.
If you are suggesting that a specific pattern system other than the ones listed https://wiki.valentinaproject.org/wiki/UserManual:Patternmaking_References needs to be supported in the documentation, please provide references to the description of the system.
The priority of the Seamly2d project is to finish the documentation of the listed patternmaking systems first.
These old books are amazing
Comment moved to - Talk:UserManual:Patternmaking References on wiki. (https://wiki.valentinaproject.org/wiki/Talk:UserManual:Patternmaking_References)