Look at any 10 by 10 snake and ladder game board, the kind that makes a good family game night, and you will see a quiet oddity. The number 100 is not in the corner above 91. It sits at the top left, directly above 81. This short piece explains why, and gives you a test you can run on any board in front of you.
How is a snake and ladder game board numbered?
You start at the bottom left with square 1. The bottom row runs left to right, from 1 to 10. Then the numbering turns. The next row up runs right to left, so 11 sits above 10, and 20 ends at the far left. The next row runs left to right again, with 21 above 20.
The name for this is boustrophedon, which is Greek for "as the ox turns". It describes how a plough goes down one row and back along the next. The same word is used for writing where alternate lines run in opposite directions. On a board, it means the path passes through every square once, and every step goes to a neighbour.
Why not number every row from the left?
Try it in your head. If every row ran left to right, the move from 10 to 11 would jump from the right edge back to the left edge. The counter would cross the whole board on a one-square step. With the ox turn, 10 is at the right and 11 is directly above it. The step from any square to the next is always a short step, and a child can follow a path by eye.
That is my explanation, not a quotation from a rulebook. It matches how the track is described, with each square touching the next.
A test for any board
Here is a check that works on the board in front of you. Every multiple of ten should have the next number directly above it. The 10 has 11 above it, the 20 has 21, the 30 has 31, and so on. If you pick a board and one of them fails, the numbering is wrong, or the board is not a standard one.
It also tells you where the last squares are. Because row ten runs right to left, 91 is at the right edge and 100 at the left. And the row below it runs the other way, so 81 is at the left, with 90 at the right.
Where do the ladders sit on that grid?
On the Classic layout I use, the biggest ladder runs from 80 to 100. Both ends sit in the left-hand column. Square 80 is the far-left square of the row below 81, and 100 is the top left corner. So the climb goes straight up the edge of the board, two rows, past 81. It looks short, but in my calculation it is the most valuable ladder on the Classic board: taking it away would add 19.77 throws for a lone counter.
The ladder from 1 to 38 is a different shape. It starts in the bottom-left corner and finishes in the fourth row, third from the left.
Seeing it on a screen
If you want to see the numbering without drawing a grid, Boardit has a free table where 1 sits at the bottom left and the board counts exactly as above.
Playing snakes and ladders online has a small advantage here. You never miscount a row, because the piece moves along the path for you, and two players or eight see the same board. A snakes and ladders board on paper is where numbering mistakes live.
Numbering your own board
When you draw a board for your own snakes and ladders game, do the bottom row first, then turn. Write the numbers in light pencil, run the multiples-of-ten test, and only then add the snakes and ladders.
Ahmed Moaz builds Boardit, a free browser game site.