Odds and Evens

Rules

Odds and Evens is played on two loops crossing at a shared point in the middle. and take turns building around either loop that still has room, 7 slots each.

Unlike Streaks, there's no choice in how many you add — the streak already sitting at the growing edge of a loop forces it. A bare loop, or one that just finished an even-length streak, forces 2. An odd-length streak forces just 1. The board shows you which applies.

Instead, your choice comes from flips. Each side starts with 2. Spend one instead of placing to flip the most recent piece on either loop — as long as it's your opponent's color and hasn't been flipped before — over to yours. That changes the streak sitting there, and with it, the count whoever plays that loop next is forced into. Once a piece has been flipped, it's locked in that color for good, whoever holds it. A faint dashed ring shows which of your own pieces the other side could flip right now, before it's even your turn.

If it's ever your turn and you have no legal move anywhere — every loop full, or every remaining loop jammed against you with nothing left to flip — you lose right there. Who moves first alternates each new game.

New to this one? Medium is a good place to start — like Streaks, it plays loosely early and tightens up later. There's less room to out-maneuver it than in Streaks, though, since so many moves here are forced rather than chosen.

Controls

The two loops are named Left and Right, matching which side of the board they're on.

With a mouse or touchscreen: tap the dashed-ring marker to place there, or the flip ring on an eligible piece to flip it.

With a keyboard: type a side into the box below the board — "left," or just "l," places there; "right" or "r" does the same on the other loop — then press Enter or Play. To flip instead of placing, put "flip" (or just "f") before the side, like "flip left" or "f r."

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

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

Like Streaks, Odds and Evens comes out of the Nim subtraction-game family — but stricter: the streak forces an exact count rather than letting you choose, and the flip mechanic is the only way to bend that. Streaks is the closer, looser sibling if you want to compare the two.

Under the hood

The two loops here don't give you a choice the way Streaks' arms do — the trailing streak at each loop doesn't set a ceiling you can choose to stay under, it forces an exact count, take it or don't move there at all. That strictness made the board size itself a real fairness problem. A capacity that's a clean multiple of 2 turns out to be a trap: an even trailing streak always forces 2 more, and 2 more of your own color is still even, so a loop with a capacity like 4 fills in exactly 2 forced moves no matter what either player does. With both loops behaving identically, the total move count across the whole board is always even — which guarantees whoever moved first is the one who eventually runs out of moves. Measured under random play, that was a 14-15% win rate for whoever went first. An odd capacity breaks the pattern, since it can jam mid-loop (one slot left, two required) instead of always landing evenly. Sweeping capacities from 3 to 9 by simulation, 7 was the first one that landed close to fair — 48.5% for whoever moves first, over 20,000 trials — so that's what shipped. The original notebook version used an odd-sized loop too, going back to childhood, though nobody remembers exactly what number — probably something close to 7. Good instinct either way: odd was the right call, it just took the simulation to confirm 7 specifically was the number worth landing on.

Flips work differently here than the strict count suggests: each side starts with 2 and spends them down over the game, never earning any back. That number was tuned too — tested across several starting budgets on the capacity-7 board, 2 turned out to be the sweet spot, enough real choice to bend an unfavorable streak without swamping the forced-count mechanic that's actually the point of the game. A flip only ever targets the most recent slot in a loop, and only if it's the opponent's color and has never been flipped before — once a slot changes hands, it's locked for good.

The AI's difficulty tiers are a direct scale-up of Streaks' own design: since the whole game solves via a cheap search that remembers every position it's already worked out, there's no "search fewer moves ahead" knob, so Easy and Medium are gated by how much of the board is left instead, just tuned for a bigger board (14 total slots across both loops, versus Streaks' 12). And the ending rule folds two very different situations into one: a loop that's completely full and a loop that's jammed (one slot left, but the streak there forces two) are treated identically — if it's your turn and there's no legal move anywhere, you lose, whichever way you got there. That strictness is also why the skill ceiling stays low: with the count forced almost everywhere except the handful of flip decisions, there's a lot less room for a stronger player to out-think a weaker one than in Streaks, where choosing which arm to play is a decision every single turn.