A classification theorem is given of projective threefolds that are covered by the lines of a two-dimensional family, but not by a higher dimensional family. Precisely, if X is such a threefold, let Σ denote the Fano scheme of lines on X and µ the number of lines contained in X and passing through a general point of X. Assume that Σ is generically reduced. Then µ ≤ 6. Moreover, X is birationally a scroll over a surface (µ = 1), or X is a quadric bundle, or X belongs to a finite list of threefolds of degree at most 6. The smooth varieties of the third type are precisely the Fano threefolds with −K X = 2H X .