As in earlier chapters, F will denote an arbitrary field of scalars, except that in this chapter, where virtually everything rests on the possibility of dividing by 2, we impose throughout the restriction char F ≠ 2 (cf. the final remark in §11). Once again, readers for whom such technicalities are unexplored territory can be assured that they will lose nothing if they simply regard F as standing for a familiar number system like ℝ or ℂ; and indeed some of the most interesting things in the chapter relate exclusively to the case F = ℝ.