Class 2 - Clock A clock allows a stalemate to appear in a few turns time, normally through pawn moves running out. It is a powerful design tool because (as with a snowdrift of pawns) it is tuneable by the problem composer for the exact setting of the clock desired. Only if there is a "tie" between two equally fast lines is there a difficulty (cook). A clock may operate either with a fixed set of moves, which when exhausted terminate the game, or with a permanent threat of stalemate, which demands continual palliation. A clock may also operate in retro, where only a fixed number of moves exist to avoid retropat for one side. See retroclock. 3N1N1k/3R1PpP/2P1B1P1/p2QP3/P1PRP3/8/8/8 w - - id #010  (15+3) DR. "J'adoube" White King to b5, b6 or c5. White to move. Last move?; 2BkN3/1R1P4/3P1R2/pKP5/P1PP4/1N6/1P1P4/2B5 w - - id #013 (15+2) DR. White to move. Last move?; NRRKBb1k/1PPPpPp1/1P2P3/6P1/6P1/8/8/8 w - - id #027 (13+4) DR. White to move. Last move?; 7k/6pP/1P4P1/1NB5/NPpP4/KpP5/1P6/b5R1 w - - id #034 (12+5) DR. White to move. Last move?; k4bBN/p1p1pPpK/PRP1P1P1/1p6/1P4P1/8/8/8 w - - id #053 (11+7) DR. White to move. Legal? Last two moves?; A more complex representation of the above idea is: rbk2b2/b1p1p1p1/KpPRP1P1/1P1p4/3P2P1/8/8/8 w - - id #053a (8+9) White to move. Legal? Last two moves? Place a White pawn on f7: Legal?; RBb2k1r/KpPp1ppp/1P1P4/1PpPp3/5p2/2P5/8/8 w - - id #065 (9+11) DR. White to move. History of the Black king?;