A quadratic line complex is a three-parameter family of lines in projective space P 3 specified by a single quadratic relation in the Plücker coordinates. Fixing a point p in P 3 and taking all lines of the complex passing through p we obtain a quadratic cone with vertex at p. This family of cones supplies P 3 with a conformal structure, which can be represented in the form f ij (p)dp i dp j in a system of affine coordinates p = (p 1 , p 2 , p 3 ). With this conformal structure we associate a three-dimensional second-order quasilinear wave equation,whose coefficients can be obtained from f ij (p) by setting p 1 = u x1 , p 2 = u x2 , p 3 = u x3 . We show that any PDE arising in this way is linearly degenerate, furthermore, any linearly degenerate PDE can be obtained by this construction. This provides a classification of linearly degenerate wave equations into eleven types, labelled by Segre symbols of the associated quadratic complexes. We classify Segre types for which the structure f ij (p)dp i dp j is conformally flat, as well as Segre types for which the corresponding PDE is integrable.MSC: 14J81, 35A30, 35L10, 37K10, 37K25, 53B50, 53C80.