  The pair $(Q, \mathscr{K})$ is a {\it knowledge space} if
$\bigcup\mathscr{K}=Q$ and $\mathscr{K}$ is closed under union, where $Q$ is a
nonempty set and $\mathscr{K}$ is a family of subsets of $Q$. A knowledge space
$(Q, \mathscr{K})$ is called {\it complementary} if there exists a non-discrete
knowledge space $(Q, \mathscr{L})$ such that the following (i) and (ii)
satisfy:
  (i) for any $q\in Q$, there are finitely many $K_{1}, \cdots, K_{n}\in
\mathscr{K}$ and $L_{1}, \cdots, L_{m}\in \mathscr{L}$ such that
$$(\bigcap_{i=1}^{n}K_{i})\cap (\bigcap_{j=1}^{m}L_{j})=\{q\};$$ (ii)
$\mathscr{K}\cap \mathscr{L}=\{\emptyset, Q\}$.
  In this paper, the existence of a complementary knowledge space for each
knowledge space is proved, and a method of the construction of complementary
finite knowledge spaces is given.
