What is a ply in chess?

A ply is one player's turn. White plays e4: that is a ply. A move in ordinary chess language is two plies, White's and Black's. Engines count in plies because plies are what they search, so an engine reporting depth 20 has looked twenty single turns ahead, which is ten moves for each side rather than twenty.

Does mate in 2 mean two moves each?

No. Mate in 2 means two moves by the side giving mate, with the defender getting one move in between, so the whole thing is three turns long. Mate in 3 is five turns. Puzzle books, composers and engines all count it the same way, which is why a mate in 5 can take a while to play out.

When is a move final at the board?

Once you have released the piece on its new square it stays there: Art. 4.7 says a piece let go on a square cannot then be moved elsewhere on that move. Before you let go you may still put it on any legal square, though touch move means it has to be that piece. After you let go, you press your clock and the turn is your opponent's.

What counts as a move in chess?

1.e4 e5 is one move. Not two. The pieces have changed square twice, but the game is one move old, and the scoresheet has one line on it. That pairing is the thing beginners trip over, because it means the word “move” does two jobs: the thing you do when it is your turn, and the numbered pair that goes in the record.

Play 1.e4 e5 2.Nf3 Nc6 from here. Four pieces have moved and the pawn on e4 faces the pawn on e5, but the game is two moves old, not four.

Ply is the word for a single turn

When you need to be exact, a single player’s turn is a ply, sometimes called a half-move. Chess writers borrowed it from computer science and engines live by it. If your app says the engine searched to depth 18, it looked eighteen turns ahead: nine moves for you, nine for the opponent.

Notation keeps the two ideas apart with dots. One dot after the number means White’s turn, three mean Black’s. So 12.Bb5 is White’s twelfth, and 12…Nf6 is Black’s reply to it. When a book quotes a black move on its own it always writes it with the three dots, which is the only way you can tell the two halves apart in running text. More of that on how to read chess notation.

Mate in 2 means two of your moves

This is the practical reason to care. A puzzle labelled mate in 2 is asking for two moves from you, and your opponent gets one in between, so three turns go on the board in total.

Mate in 2. The rook check 1.Ra8+ fails to 1...Kh7, so first the king takes h7 away: 1.Kg6, and the black king has only g8. Then 2.Ra8 is mate.

Count the turns in that solution: White, Black, White. Two white moves, and the second one ends the game. Get used to counting that way before you start on mate in two puzzles, because looking for a two-move solution and forgetting that Black gets a say in the middle is how people fail them.

The fifty-move rule counts pairs, and so does the clock

The draw claim in Art. 9.3 needs the last fifty moves by each player to have gone by without a capture and without a pawn move. That is a hundred turns, not fifty, so it is twice as long as most people assume and no threat at all in ordinary endings. Learning the fifty-move rule properly saves you from claiming a draw at move 60 and getting a two-minute penalty for a wrong claim.

Tournament time controls count in the same units. “90 minutes for the first 40 moves” means forty of your own turns, and the extra time appears the instant your fortieth is on the board.

When your move is a move

Two moments matter at the board. The first is releasing the piece: under Art. 4.7 a piece let go on a square cannot then be sent anywhere else on that move, so the fingertip on d4 is the point of no return. The second is pressing the clock, which hands the turn over. In between, the move already counts, and the only thing left to do is press.

That is why you should decide, then move, then press, and never the other way round. Sitting with your hand hovering over a piece is how a rook ends up on the square your finger picked rather than the one your head did.