A threat to capture the opponent's king does not give check (ie, it could not
actually be carried out) if it opens one's own king to immediate recapture.
Consequences
First, this means that attacking the opponent's king is a valid escape from
check. In a situation where both sides have pieces attacking the other king, then
neither side gives check, because if one side carried out the king capture, the
other side could immediately match it.
Second, it means that a protected king can attack the opposing king, provided
no other opposing piece also attacks it. This has profound endgame
consequences, because it means that K+B vs. K and K+N vs. K are at the very least winnable
as helpmates. (Whether they can force mate is another question). If played
with non-standard pieces, K+(any piece that can protect it) vs. K is winnable as
a helpmate.
Written by Robert Shimmin.
WWW page created: June 30th, 2003.