Triangulation for life drawing

Triangulation is as old as geometry itself and its uses are without end. However triangulation is strangely absent from books on drawing, so much so that for a long while I thought (though I found it extremely unlikely!) that I was the first to use it for such purpose! (Oh, vanity!) Alas, there is nothing new under the sun, and recently I found it explicitely used and named as such in a 1971 book by Douglas Graves, though he didn't go into much detail (and a little while later found a couple of other sources on the web, though they used it in a more two-dimensional context, basically for drawing from photo reference - and 3D poses other challenges).

Claims to originality being thus shattered, it still remains true that I thought about it at lenght and used it and taught to colleagues for a few years, so I think I have something useful and maybe new to stress on its practical uses and caveats, so in a coming series of posts I will try to expound on triangulation - how and why to use it in drawing from life. I hope you may find this useful.