Four centre ellipse — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
A ruler on an isometric drawing
Isometric drawing has one scale — 0.8165 — and every account of it stops there. But that number is about three directions and a drawing has infinitely many, so a length measured off the paper and divided by 0.8165 comes back anywhere between √(1/2) and √(3/2) of the truth: 29.3% short to 22.5% long, with nothing in the picture to say which.
The ellipse the drawing office draws
Every isometric ellipse template is cut to a construction of four circular arcs, and every account of it calls the result an ellipse. It is not one and cannot be: a circular arc has constant curvature and a conic's varies, so the two can agree at four points and nowhere between. The four-centre curve reaches 2√2/3 of the true semi-major axis — 5.72% short — and its minor axis is 3.53% too long.
Named alongside it
The objects these essays reach for when they reach for this one.
AnisotropyDrawing systemForeshorteningIsometricOrthographicTaught and unmeasuredConicDemonstrationinstrument limitOblique projectionPlanometricReference length