  We describe a "concentration on the diagonal" condition on the Khovanov
complex of tangles, show that this condition is satisfied by the Khovanov
complex of the single crossing tangles, and prove that it is preserved by
alternating planar algebra compositions. Hence, this condition is satisfied by
the Khovanov complex of all alternating tangles. Finally, in the case of links,
our condition is equivalent to a well known result which states that the
Khovanov homology of a non-split alternating link is supported on two
diagonals. Thus our condition is a generalization of Lee's Theorem to the case
of tangles
