Pearson's r = ∑(x − x̄)(y − ȳ) / √(∑(x − x̄)² ∑(y − ȳ)²) adds one signed rectangle per point, spanned from the point to the mean centre (x̄, ȳ). Points above right or below left of the centre add area; the other two quadrants subtract it. By Cauchy-Schwarz, −1 ≤ r ≤