Checkless Chess (or prohibition chess) is a chess variant where neither player is allowed to give a check, with the exception of checkmate. All other rules are as in regular chess. This change has a profound impact on the way the game is played.
In regular chess, the king needs to be kept safe, since attacks on it need to be parried, and checks can be used to gain time or chase the king to an unsafe position. In checkless chess however, the king is immune from most attacks, as long as checkmates are avoided. In order to achieve checkmate the king must be encircled without checks.
Another impact of this rule is that the king, immune from attack, is now itself a powerful force. The king can defend pieces by placing itself so that capturing the piece would place the king in check. The king can advance into the enemy position, creating havoc in the enemy camp as they need to avoid squares where they would put the king under check. Such a plan may be risky however, since getting the king trapped in the enemy camp may subject it to an untimely checkmate.