Explanation to the Above Figure
K is checking K if K is moved to x or y. K could not capture K because it is blocked by both knights. Please reread the definition of the king carefully if you have a hard time understand this.

New Concepts
Five of the fourteen new concepts that were developed and used exclusively for the creation of xxChess (QTChess, BDChess, and UBChess) are listed below. Not all of these concepts are used in every game.

1. The concept of duelling kings, as manifested in the nature of the xxChess kings, allows its kings the natural ability to give each other check. See Figure-11 above. In this example, a possible king-checking-king is shown: one king(K) is able to check another king(K). This ability is inconceivable in other chess versions or variants when both kings' nature are identical.

2. The concept of maximum promotion, as manifested in the xxChess pawn when it reaches the last row, simplifies the relationship between the pawns and the king toward the goal of the game.

3. The concept of no-sub-set discourages the nature of redundancy by not allowing any piece to be a sub-set of another piece. According to the concept of no-sub-set, the co-existence of the (FIDE) chess king, rooks, and bishops are in violation because these three pieces are each a sub-set of its queen. One solution would be the elimination of the (FIDE) chess queen.

4. The concept of safe blocking allows a piece to safely block a threat coming from the rook, the bishop, the elephant, and the king.

5. The concept of advancing/retreating makes it possible to safely block a rook, a bishop, or an elephant, which allowed about 33% more tactics to the effected pieces. Thus when a piece is threaten, there are four, instead of three, possible responds: 1. capture the threat, 2. run away from the threat, 3. set up a retaliation, or 4. block the threat.

6. The concept of Western compatible requires the game to have exactly six types of pieces: king, queen, bishop, knight, rook, and pawn.

7. The concept of Vietnamese compatible requires the game to have exactly seven types of pieces: king [tuo'ng], queen [si~], bishop [pha'o], knight [ma~], rook [xe], elephant [voi], and pawn [cho't].

8. The concept of (FIDE)chess compatible means a set of (FIDE)chess could be used to play the new game.

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