A projection of a vector space V is a linear map P:V→V such that P2 = P. Then V is the direct sum of the kernel and image of P. Conversely, if V = M⊕N, every v can be uniquely written as v = m + n (where m ε M, n ε N), and the map v↦m is a projection. If V is an inner product space, then V = M⊕M⊥ and v↦m is orthogonal projection onto M.