Below 299: Learn basic rules of chess, how pieces move, and special rules (en passant, castle etc).

1000-1199: Learn basic opening ideas. It's recommended to play 1.e4 as white and sharp variations as black. Learn basic checkmates (King + Queen vs. King, Queen + Queen vs. King, Rook + Rook vs. King, King + Rook vs. King). Practice them until you are completely confident and can checkmate anyone (even a GM) in these positions.

1200-1399: Study more openings, but do not stick with "rarely played variations". It is a huge mistake that a LOT of chess player make while studying openings, to study rarely played/unusual lines which most likely would never come up in real life tournaments. Studying standard opening lines would yield a lot more results!
Spend maximum of 20% of your study time to study openings. Until 1800 level openings aren't very important. Concentrate more on middle game and tactics.

1400-1599: Endgame is the key on this level. Only imagine the advantage you get against your opponent if you know how to play endgame well and your opponent does not. The odds are that your opponent will loose the endgame almost immediately. Study basic endgame schemes: King + Pawn vs. King, King + 2 Pawns vs. King, King + Pawn vs. King + 2 Pawns and so on. Besides the endgame do tactics problems (puzzles). Play as much chess as possible especially in real life over the board tournaments with a long time control (that's where you learn the most).
1600-1799: If you got on this level it means you are already strong, very solid chess player which knows a lot about all stages of the game of chess. Keep working on your middle game, strategy and tactics. Try to play blindfold chess. It is easy these days, since programs like Chessmaster 10th offer really nice options of blindfold chess. You are basically only see the chess board with no pieces on it, but you can move these empty squares and play chess (because you know that knights are on b1 and c1, bishops are on c1 and g1 and so on). During the game you have to keep all the pieces in your head, since they are invisible which is hard in the beginning. Play really weak players first.
Blindfold chess helps to develop player's visualization ability and eliminates most blunders. All GMs are able to play blindfold chess well.

1800-1999: Wow, you are ready to become an expert. Now it is a good time to systemize your opening repertoire. Ideally, you should know very well 2-3 openings for white and 2 for Black. The key to advancing to the expert level here is to analyze your own games. After the game, sit down with a chessboard, paper and pen and go over the game move by move writing down thoughts on your own and your opponent's moves. Only then you may check your game with an engine (Rybka, Fritz, etc). The number #1 mistake chess players make, they either do not analyze their games at all or analyze them by using chess program right away. This is a big mistake which slows down chess progress.The main idea of analysis is not to look at what computer thinks was a good move, but to look for that move yourself! Would it make sense to solve chess tactics puzzles by plugging them into a chess engine? Probably not.Also it is great idea to go over GM games and think them over. The best way to go over these high level games is to first go over all the moves without author's comments and only second time read the comments and annotations. This works great with the openings too. Remember that when you read your opening book tomorrow.

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