What Does f[subscript xx]f[subscript yy] - f[superscript 2][subscript xy] Greater than 0 'Really' Mean?

This note presents geometric and physical interpretations of the sufficient condition for a critical point to be a strict relative extremum: f[subscript xx]f[subscript yy] - f[superscript 2][subscript xy] greater than 0. The role of the double derivative f[subscript xy] in this inequality will be highlighted in these interpretations. (Contains 14 figures.)