Mate in Five, Then Mate in Seven
A mate score replaces the evaluation entirely, and it moves around as the search deepens. What the number counts, and why a longer mate is not worse news.
· 3 min read
When an engine finds a forced mate, the pawn evaluation disappears and is replaced by a mate score. The reason is straightforward: material has no exchange rate against checkmate. Being a queen down while mating in two is not worth minus nine, it is worth the game.
What the number counts
Moves, not plies. M5 means five moves by the mating side, so up to nine or ten individual moves are played before it ends. This trips people up when they count the line and find more moves than the number promised.
The sign, where one is shown, indicates who is mating, from White's point of view like every other evaluation: +M5 means White mates in five, −M5 means Black does, whoever is to move. The raw output of most engines reports mate from the side to move instead, which is why the same mate can appear with the opposite sign elsewhere; this site converts it so the sign always means the same thing.
Why the number moves
| What you did | What the score does | Is it bad? |
|---|---|---|
| Played the fastest mating move | Counts down by one | No |
| Played a different winning move | May go up | No, still forced |
| Played a move that lost the mate | Reverts to a pawn score | Yes |
| Nothing, but the search deepened | May go down | No, it found a faster line |
Only the third row is a problem. A mate in three becoming a mate in six after your move means you took a slower route to the same result. The game is equally over.
This is worth knowing because game reviews sometimes flag the slower path as a mistake, having measured it as a loss of value. In practical terms, a forced mate in six that you can see is better than a forced mate in three that you cannot.
Shallow searches announce mates that are not there
A mate score from a low depth is a claim, not a fact. The search may have found a line where every reply it examined loses to mate, without having examined the defence that holds.
The tell is a mate score that appears and then vanishes as depth increases, replaced by an ordinary evaluation. That means the first announcement was premature. A real forced mate does not go away with more searching; it gets shorter or stays the same.
If a mate score matters to you, search deeper and watch whether it survives. Ten seconds settles it.
Mate scores in the endgame
The most reliable mate scores come from tablebases rather than searching. For positions with a small number of pieces, the result of every possible position has been computed exhaustively and stored, so a tablebase does not estimate anything. It looks the answer up.
An engine consulting a tablebase will report a mate in thirty-eight without hesitation, and it is correct, though the line will be incomprehensible. Those distances are why the fifty-move rule occasionally forces a draw in positions that are theoretically won: the win exists and takes longer than the rules permit.
When a mate score should worry you
There is one case where a lengthening mate is genuinely bad news, and it is worth separating from the harmless kind.
If the mate distance grows while you are following the engine's own recommendations, something is wrong with the analysis rather than with your play. Usually the earlier, shorter mate came from a search that had not verified a defensive resource. The honest reading is that the position is winning and the exact distance was never known.
If the mate score disappears entirely and is replaced by a pawn evaluation, the mate was never there or you have just given it away. Check which by stepping back one move and re-analysing the previous position at greater depth.
Reading a mate line
The line an engine gives for a mate is a proof, not a plan. It shows one sequence in which every defence loses, and it is often not the sequence a human would choose, because the engine has no preference for lines that are easy to remember.
If you want to play the mate rather than admire it, look for the move that forces the most and branches the least, even if it takes a move or two longer. A mate in seven you can execute beats a mate in five you will lose track of on move three.
Common questions
- What does M5 mean in chess analysis?
- Forced mate in five moves, whatever the material on the board says. The sign says who is mating, from White's point of view like every evaluation: +M5 is White mating, −M5 is Black. It replaces the pawn evaluation because mate ends the game, and no quantity of pawns is comparable to that.
- Why did the engine say mate in 3 and then mate in 5?
- Usually because you played a move that was not the fastest path, which is normal and harmless. It can also mean the earlier score came from a shallower search that had not verified the whole line.
Try it on your own position
The analysis board runs the same engine in your browser, free. The app adds what a web page cannot: reading a position straight from a photograph, saved boards, and analysis with no connection.
