Can a bishop change colour?

Follow a bishop for an entire game and it never once changes colour. The reason is arithmetic, not a special rule: every step of a diagonal moves one file across and one rank along, so a bishop on c1 goes to d2, e3, f4 and so on, and each of those squares is dark. The bishop that starts on c1 will still be on a dark square when you resign. The bishop on f1 will still be on a light one.

That single fact drives a surprising amount of chess.

Half the board, all game

Each bishop sees 32 squares and is blind to the other 32. Give it two moves and it can reach any square of its own colour from anywhere, which makes it fast, and it can never in its life defend a pawn standing on the wrong colour, which makes it one-eyed. Two bishops together cover everything, and that is why the bishop pair is worth a real bonus in an open position.

The endgame is where the blind spot bites.

8

7

6

5

4

3

2

a

b

c

d

e

f

g

h 1

Look at what is happening there. The pawn wants h8. The bishop covers g8 and stops the black king coming out, but it cannot cover h8 and it cannot make the king move off it, so White’s only way to force the issue is stalemate. This is the famous wrong bishop, and it turns up in real games often enough that every coach teaches it: with a rook’s pawn, check the colour of the queening square before you trade into the ending.

Pawns on your own colour

The other cost is quieter. Put your pawns on the same colour as your bishop and they get in its way, and the bishop spends the game staring at the back of its own infantry. Put them on the opposite colour and the bishop breathes, though the squares the pawns left behind are now yours to worry about. That trade-off has a name and a page of its own: good and bad bishops.

The French Defence is the classic case. After 1.e4 e6 2.d4 d5 Black’s pawns are both on light squares, and the bishop on c8 spends the next twenty moves looking for a way out. Whole systems exist just to solve that one bishop.

And when a position closes up and the pawns get fixed, colour is what decides whether you want the bishop or the knight. A knight visits every square on the board. Your bishop, whatever you do, gets half.