Corner 'Round

Rules

Corner 'Round is played on 4 rings around a center dot, each ring split into 8 lanes (like slices of a pie). Two players — and — take turns dropping a marker into any of the 8 lanes. A dropped piece falls to the innermost open spot in that lane (closest to the center dot).

Get 4 in a row in any of these three ways to win:

• Ring: 4 in a row around the same ring (wraps around the circle)
• Lane: all 4 rings filled in the same lane, straight out from the center
• Spiral: 4 in a row stepping outward one ring at a time while also turning one lane over, same direction each step

If all 32 spots fill with no winner, it's a draw.

Aggressive difficulty plays differently from the others: it only ever defends to stop your immediate win — otherwise it ignores you completely and just pushes its own furthest-along line. It's beatable in ways Hard isn't, but it won't hand you a win either.

New to this one? Medium is a good place to start — it plays like a genuine intermediate opponent, not just a weaker copy of Hard.

Controls

Lanes are numbered 1 through 8, clockwise around the board. Rings are numbered 1 (innermost, next to the center dot) through 4 (outermost, the rim).

With a mouse or touchscreen: click any open wedge in a lane, or the arrow button on the rim, to drop a piece there. It falls to the innermost open ring in that lane.

With a keyboard: type a lane number into the box below the board — "3," for example, drops into lane 3 — then press Enter or Drop.

Type status (or just the start of it, like "sta") and press Enter or Drop at any time — even when it isn't your turn — to have the whole board, ring by ring and lane by lane, read back to you.

Type explore (or just "e," "exp," and so on) and press Enter or Drop to open Explore This Game — the same as clicking the Explore This Game button in the toolbar — reachable during or after a game vs the computer. Once you've run Grandmaster analysis there, typing a move number and pressing Enter or Drop jumps straight to the board position right after that move, the keyboard equivalent of clicking that move in the analysis list.

Don't want the keyboard box taking up screen space? There's a "Keyboard move-entry" checkbox under Settings that hides it — on by default with a mouse or trackpad, off by default on touch, and remembered separately for each device you play on.

Roots

Corner 'Round is part of a genre of vertical games such as Score Four and Connect Four. You drop a piece and it settles as far as gravity (or here, the ring's pull toward center) lets it go, rather than landing wherever you click. Connect Four itself traces back to much older "align four in a row" folk games and the ancient n-in-a-row family that also includes Gomoku.

If you want the straight version, you can play Four-in-a-Row in Classics Corner.

Under the hood

The four difficulty levels aren't a slider on one brain — each plays a genuinely different way. Easy is intentionally forgetful: it only checks for its own winning move half the time, and even when it goes looking for a block, it's blind to the trickiest of the three win shapes (the spiral), so it gets caught off guard the way a beginner would. Medium always takes a win and blocks a threat it notices (80% of the time), but only thinks one move ahead. Hard sees everything and runs a full minimax search six moves deep, with alpha-beta pruning cutting off branches of the search tree it can prove won't matter, helped along by move ordering — checking the most promising moves first so that pruning kicks in earlier. Aggressive is its own animal — it will always stop your immediate win, but otherwise ignores you completely and just greedily extends its own longest line, so it can walk right into a fork you set up two moves earlier.

Medium wasn't always a one-move thinker. It used to run the same minimax search as Hard, just three plies deep instead of six — and in testing, two Medium AIs playing each other drew all 40 times out of 40. That much lookahead let a merely-good player always find a safe reply to everything. Dropping it to a single ply — it kept the automatic win and the usual block — produced zero draws in the same test, while still losing cleanly to Hard and beating Easy cleanly — a smaller search by design, tuned to play like an actual intermediate opponent instead of a shallow-but-exhaustive one.

Is Corner 'Round solved? No — and not for lack of trying elsewhere on this site. Four-Patch, one of the other originals here, does have an exact, proven solution, but its solver needed a transposition table — a cache of positions it's already worked out, so it never re-solves the same one twice — layered on top of alpha-beta pruning just to make the full search tractable. Corner 'Round doesn't have a solution like that anywhere in this project, because its three win shapes — rows, radial lanes, and spirals — overlap in ways that haven't collapsed into a simple strategy, even for a search that went looking. That's the real reason the skill ceiling stays high: getting better keeps paying off, because nobody, including this site's own search, has found the bottom of it yet.