Main Page

Welcome to the explain xkcd wiki!

We have collaboratively explained 6 xkcd comics, and only 3171 (52850%) remain. Add yours while there's a chance!

Latest comic

Go to this comic explanation

Chessboard Alignment

|< < Prev #3177 (December 5, 2025)

Title text: Luckily, the range is limited by the fact that the square boundary lines follow great circles.

|< < Prev Random Explanation

Explanation

The comic shows an overhead view of three chess boards side by side, with two players facing each other across most of the boards. Yellow squares (used to show the available or actual movement of a given piece) have been marked leading from the starting position of the middle board's right bishop (F1) to the upper-right. The path continues beyond the edge of the middle board, across four columns of empty space or unseen table, and ends in the top left corner (A8) of the right board. The right board has only one rook (black rectangle) while the other two boards each have two, so it is implied that the bishop has captured the rook. The text below jokingly claims that if you align chess boards exactly, pieces can cross the boundary like this. This is not legal in normal chess ^{[citation needed]}, but fits into Randall's long history of comics about unusual chess rules or boards.

The title text refers to the fact that chess boards are normally placed approximately level (parallel to the surface of the Earth). A perfect line of chessboards placed end to end on the surface of an Earth-sized sphere (or on perfectly placed tables on that sphere) would form a "great circle" - the longest possible path around that sphere. While nearby boards would appear to be in the same plane, the curvature of the earth would cause boards more distant than 3.57 meters away to be in planes so different that the squares would be more than a micrometer off from the ideal straight lines leading off the board. It is thus implied that each infinite-range piece's valid path is a straight line of virtual squares that eventually leads into space. Otherwise, the alleged rule would allow chess moves between boards that were kilometers (or even whole countries) apart in any vertical line. If following the great circle along the ground was considered a straight line, then it would also be possible for each side's rooks and queen to capture their counterparts in the other color's back row, just by moving backwards around the planet. This does not rule out motion to another board on another celestial body or spaceship, though delivery of a chess piece across this distance would be impractical^{[citation needed]}. This is thus the second comic in a week about distances extending past typical boundaries.

Transcript

It doesn't happen often because it requires micrometer precision, but if two chess boards are perfectly aligned, it's actually legal to move pieces between them.

Is this out of date? .

New here?

You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

• If you're new to wikis like this, take a look at these help pages describing how to navigate the wiki, and how to edit pages.

• Discussion about various parts of the wiki is going on at Explain XKCD:Community portal. Share your 2¢!

• List of all comics contains a complete table of all xkcd comics so far and the corresponding explanations. The red links (like this) are missing explanations. Feel free to help out by creating them! Here's how.

Rules

Don't be a jerk. There are a lot of comics that don't have set in stone explanations; feel free to put multiple interpretations in the wiki page for each comic.

If you want to talk about a specific comic, use its discussion page.

Please only submit material directly related to —and helping everyone better understand— xkcd... and of course only submit material that can legally be posted (and freely edited.) Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.

If you need assistance from an admin, feel free to leave a message on their personal discussion page. The list of admins is here.