Chess vs Computer
Chess against the computer in the browser, on a phone, tablet or computer, or pass-and-play with two people on one device. You pick one of five opponents instead of climbing a ladder, and an optional coach marks each of your moves and says why. The School, a page of its own, drills openings, tactics and endgames.
How to play
Tap a piece, then tap where it goes. Legal squares are dotted, and a square you can capture on is ringed in red. A king in check turns red — you must deal with it.
Everything is here: castling (move the king two squares), en passant, and under-promotion — a pawn reaching the far rank asks what it should become rather than assuming a queen.
Draws are called for you: stalemate, the fifty-move rule, a third repetition, and material that cannot mate. UNDO takes back your move and the reply.
The opponents
BEGINNER sees the piece you just hung, about half the time. CASUAL takes free material and defends the obvious. CLUB counts the trade before it starts it. STRONG gives away no free pieces and punishes a loose one within a move or two. EXPERT thinks properly, and takes its time doing it.
A chess game is half an hour, so losing one does not send you back to a beginner. Wins, losses and draws are kept for each opponent separately, in your browser.
The computer thinks in the background, so the board keeps drawing while it looks ahead, and the stronger opponents are allowed a few seconds.
The coach
Turn on COACH and four aids become separate switches. THREATS lights up your pieces that are under-defended and enemy pieces you can win. GUARD asks if you are sure before a move that hangs material. HINT draws the engine’s best move as an arrow. IDEA offers one plain principle for the position.
Every move you make is graded BEST, GOOD, INACCURACY, MISTAKE or BLUNDER by comparing it with the best move available, with a reason. Takebacks are allowed and counted. After the game your opening gets a letter grade.
There is deliberately no evaluation bar. Tapping WHY? gets a fuller explanation, written by a language model that is handed the engine’s facts so it explains the position instead of inventing one.
The School
A separate page of drills. Five openings, each played move by move against the book with the reason for every move: the Italian Game, the Queen’s Gambit, the London System, the Sicilian Defense and the Ruy Lopez. Watch the line first, then play it from memory.
Five tactics, the fork, the pin, the skewer, the discovered attack and the back-rank mate, with ten find-the-move puzzles between them. And four endgames played against the engine at full strength: mate with a queen, mate with a rook, the opposition, and the square of the pawn.
Checked against the numbers
Every candidate move is played out and thrown away if it leaves your own king attacked. It is the slow way round, and it means pins, discovered checks and castling through check all come out right from a single test.
The move generator is tested against the six standard counting positions chess programmers use, each designed to catch a different mistake, such as forgetting that capturing a rook on its home square removes that side’s castling right.
The engine also keeps looking at captures until the trading stops, so it never judges a position halfway through an exchange.