The maximum and minimum normal curvatures at a point on a regular surface are called the principal curvatures at , usually denoted and , respectively. The corresponding directions, and , are called the principal directions at .
The principal curvatures and directions have an inuitive interpretation in terms of the second fundamental form (sec:SecondFundamentalForm) -- the principal directions are its eigenvectors2.5. The second fundamental form (and thus the normal curvatures) takes its minimum and maximum values at the eigenvectors, and those values are the negative of the eigenvalues. (The negative sign comes from the definition of the second fundamental form: .)
The principal directions form a convienient vector basis for determining curvatures of any direction in the tangent plane. If is an arbitrary unit vector in the tangent plane, then clearly , where is the angle between and . Working from the definition of the second fundamental form, we have