The longest possible chess game

But you get stuck

Because before black can promote, white needs to move at least 1 pawn.

But 5900 is a good answer. I think that was my answer the first time I saw this puzzle about 10 years ago.

It's close, but it's not right.

Hmm... Then 3 switches at least. Move 4 white pawns, switch #1 to the black.

Move all black pawns and open 4 files, switch #2 back to white.

Move all white pawns to the end, switch #3 to black.

Complete black pawns moving.

Hmm.

First of all black has to start, but that's a minor detail (I can just pretend the colors on your boards are reversed).

The real problem is your 2nd diagram. Black captured 4 of white's minor pieces to do that... but there was no way to capture white's light squared bishop.

That might sound like a minor complaint, but I think because of the limited number of captures (at first you can only capture the opponent's knights until you switch to them) this sort of thing will increase the number of switches by 1.

So I think your way of doing it would have 4 switches. Each switch subtracts half a move...

So 5900 - 2 = 5898

"... there was no way to capture white's light squared bishop."

Oops))) Sorry for this))) Ok, it can be not the bishop but the rook or queen.

And what about 75 moves rule? Earlier I hadn't heard about this, but if it's true then the number of moves increases up to 118*75=8.850-2=8.848 (as Everest)

But in the beginning you can only capture your opponent's knights. So you only have 2 captures available for the side who starts. That's why it adds a switch... so like you said, 3, but I'm saying + 1, so I think 4 switches.

As for the 75 move rule, yes

But I think chess.com automatically claims it on move 50 now

I wanted to try with a friend a few years ago to make the longest game on chess.com, but we gave up after a while... also even if your game is 2000 moves long, chess.com stops recording moves after 500 (if I remember correctly). So in your game history, and even when you go to download the game, it will only be 500 moves.

By the way, when this was originally asked, a lot of people worked on the solution, and after a while, when they reached the number 5900 - 2 = 5898 they thought it was correct, and the topic didn't get much attention for a few months...

But all of those people overlooked something. After those months someone posted a way to only make 3 switches

Not only the knigts, below is example how black can take other pieces.

Ok, lets look at it.

Phase 1
Current state:
Zero pawns have made a capture
Zero switches

Description:
Black advances all his pawns to the 5th rank.

Phase 2
After phase 2:
Zero pawns have made a capture
1 switch

Description:
White advances his pawns

Phase 3
After phase 3:
4 pawns have made a capture
2 switches

Description:
Black promotes 4 of his pawns and captures all of white's pieces

Phase 4
After phase 4:
8 pawns have made a capture
3 switches

Description:
White promotes all his pawns and captures all of black's pieces.

Phase 5
After phase 5:
8 pawns have made a capture
4 switches

Description:
Black promotes his last 4 pawns, and captures all of white's pieces.

Phase 6
After phase 6:
8 pawns have made a capture
5 switches

Description:
The white king capture the last of black's pieces

96 pawn moves + 30 pieces to capture = 126
- 8 times the pawns made a capture = 118
x50 = 5900
- one half for each of the five switchs = 5897.5

Of course, the above can be reduced from 5 switches to 4 switches if you start with this (the phase of black advancing his pawns to the 5th rank was not necessary).

And so the current solution is 5898

However, I made the claim that someone in the past (not me!) figured out how to do only 3 switches.

If I'm telling the truth, then the longest game would be half a move longer... 5898.5

He's wrong.

Like I said, 5898 was what a lot of people thought when I first watched them try to solve the puzzle on these forums (around 10 years ago or so). A few of these people were really smart...

But then someone came along and improved it to 5898.5

Here's a hint... what's everything black can do during the first phase, and how can that help him? Try to make use of all his options. For example in post #31 above I made black set up his pawns like that.

But what else can black do, and how can it help?

Alternate after phase 3: 

4 pawns made a capture 

2 switches

Desc: black blocks 4 of white’s pawns and promoted 4 pawns (3 to queens, 1 to knight, Black’s promoted pieces captures white pieces except for pawns

Something like this

too bad the analysis board doesn't understand repetition or 50-move rule, i could experiment with it forever

The trick is, while black is moving first, he can do more than move pawns, he can capture both of white's knights.

So then you have to ask, but if you're only opening 2 files instead of 4, does that actually make a difference?

The answer is yes. Instead of each side opening 4 files, you can have black open 2, and then have white make the other 6 pawn captures.

First phase black sets up this pawn structure

Now the turns switch and it's white to make captures and pawn moves. During this phase he promotes all his pawns and capture all of black's pieces.

Now it's black's turn to make pawn moves and captures again. He promotes all his pawns and captures all of white's pieces.

And finally the last phase. White captures all of black's remaining pieces.

And that's how you can do it with only 3 switches, and so the longest game with 50 move rule is

(118 x 100) - 3 = 11797 ply or 5898.5 moves (the game ends on white's 5899th move)