Unlike the first fundamental form, this construct appears in the definition of the coefficients of the second fundamental form. Negative torsion multiplies this construct in one of the Frenet-Serret formulas, (-5[1])which relates it to a construct whose name prepends "bi-" to this construct's name. This construct points toward the center (10[1])of the osculating circle. A choice of two possible signs (10[1])for this construct determines (-5[1])the orientation of a surface. One of these constructs and an arbitrary point on the plane yield the simplest vector (10[1])definition of a two-dimensional plane. If a line has slope m, this vector points along a line of slope "negative one over m." (10[1])In two dimensions, this vector (-5[2])is defined by being at 90 degrees to the tangent vector. For 10 points, name this vector that, at a given point, is perpendicular to a curve. ■END■ (10[2]0[3])