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To have a good (near invisible) junction between two triangular zones means succeeding, with different lines boldness and angles, to cover the paper with the same proportions of colors. If it is visually good, it means that the drawing instrument is well calibrated, and that all the calculus were correct.
In my next drawing D65 n°6 I will have a lot of junctions like this one, the challenge will be to make them all as good as this one! :)