The set of all planes through a line. The line is sometimes called the axis of the sheaf, and the sheaf itself is sometimes called a pencil (Altshiller-Court 1979, p. 12; Gellert et al. 1989, p. 542). The equation of a sheaf of planes specified in Hessian normal form is ((n_1)^^·x + p_1) + λ(n^^·x + p_2) = 0.