Dyadic rational — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
Seven is not a power of two
Halving a receding depth by diagonals is exact, and every book gives it. Halving reaches a half, a quarter, three eighths — and never a third, however many times it is spent, because no power of two is divisible by three. There is a construction that reaches every whole fraction, it costs three lines rather than a stack of quadrangles, and the extra ingredient is not a measurement.
The circle in the square wants a number
Every manual draws a circle in perspective by inscribing it in a square, crossing the diagonals, and marking four more points about seven tenths of the way out. Seven tenths is right — along the diagonal of the real square. Along the drawn one it is 54 mm off a circle two and a half metres across, and the drawn diagonal is the only diagonal on the paper.
Named alongside it
The objects these essays reach for when they reach for this one.
Cross ratioHarmonic conjugateStraightedge constructionAffine structureComplete quadrangleConditioningConicForeshorteningHorizonIncidenceInscribed circlePicture plane