Overview

Number Baseball

1 player · 2-8 min per session

Choose your own hidden number, then crack the opponent's secret before the computer fully decodes yours. Each mode has a fixed try limit, and if nobody reaches a full strike count before both sides run out of guesses, the round ends in a draw.

Players: 1P Session length: 2-8 min

NumberPuzzleStrategyBoard Game

Goal & Core Rules

VS Computer modes begin with a secret-setting phase: you lock in your own number first, then the computer prepares a different secret and the duel starts.
Easy and Normal use 3-digit secrets, while Hard uses 4 digits. Every secret and every guess must use unique digits, and the first digit can never be 0.
After each guess, the game reports strike-ball feedback: a correct digit in the correct place is a strike, a correct digit in the wrong place is a ball, and 0 strikes plus 0 balls is shown as OUT.
Turns alternate between you and the computer. Both sides keep separate history panels, so you can track not only your own progress but also how close the computer is getting to your secret.
You cannot submit the same guess twice, and invalid inputs are blocked before or at submission. The round ends immediately when either side hits the full strike count.
If both sides use up all tries without a full solve, the game declares a draw. The build also includes Computer vs Computer spectator modes that run the same deduction rules automatically.

Current implementation: head-to-head code duel

This is not the most common single-secret classroom version. You first choose a secret that the computer must read, the computer hides a different secret, and both sides alternate guesses. VS Computer Easy uses 3 digits with 10 tries, VS Computer Normal uses 3 digits with 9 tries, VS Computer Hard uses 4 digits with 10 tries, and the two Computer vs Computer modes let you watch the same race without manual input.

Common notebook version: one hidden number

A lot of people learn Number Baseball as a simple paper game: one side writes a secret and the other side keeps guessing until it is solved. There is no counterattack, so you only care about your own deduction speed. Compared with that familiar format, this build feels much more like a duel because every turn that teaches you something also gives the computer another chance to read your number.

Common house rules: 3 digits vs 4 digits

Korean casual play is often taught with 3 distinct digits, while international Bulls and Cows examples often use 4. Some groups also change the try limit or debate whether 0 is allowed anywhere. This build keeps the first digit non-zero in every mode, stays with 3 digits in Easy and Normal for a faster match, and expands to 4 digits only in Hard and Computer vs Computer Pro.

Common house rules: duplicates and OUT calls

Some versions allow repeated digits, and some simply report 0 strikes and 0 balls without using the baseball word OUT. Repeats make clue reading much trickier because one digit can match more than once. This build keeps the deduction cleaner: every code must use unique digits, repeated guesses are rejected, and a total miss is labeled OUT right away.

Controls

Mouse

Click the on-screen number buttons to fill the secret or guess slots. The first slot rejects 0, and duplicate digits are blocked.
Click the Recommend button to auto-fill a legal secret during setup or a suggested guess during the duel.
Click <- to erase the last digit, Clear to empty the whole input, and Start Match or Submit Guess to lock in the current number.
Use the top menu for restart, help, and mode changes. In Computer vs Computer modes, you do not enter numbers at all - just watch the spectator panel and both history columns.

Keyboard

There are currently no dedicated keyboard shortcuts. On desktop, use the on-screen keypad and action buttons.

Touch

Tap the on-screen number buttons to fill the secret or guess slots. The first slot cannot be 0, and duplicate digits are blocked.
Tap the Recommend button for a ready-made legal secret or a suggested next guess.
Tap <- to erase the last digit, Clear to wipe the whole input, and Start Match or Submit Guess to confirm the current number.
Use the top menu for restart, help, and mode changes. In Computer vs Computer modes, simply watch the duel without entering input.

Beginner Tips

In this build, your own secret matters too. Do not stare only at your guesses - watch the computer's history and how quickly its list of possible answers is shrinking.
An OUT is still valuable information. A total miss removes several digits at once and can be better than a vague 0 strike / 1 ball clue.
Before you submit, ask what the guess will teach you even if it fails. Good information usually beats a flashy hunch.

Advanced Tips

Use the Recommend button as a baseline, not a command. When two patterns seem close, choose the guess that separates them best, even if it is not the most tempting answer at first glance.
Because either side can win first, tempo matters. Once you already know part of the code, avoid spending too many turns on tiny improvements.
In 4-digit Hard and Computer vs Computer Pro, the search space is much larger. Early guesses that split the field broadly are more useful than all-in gambles.
If the computer is clearly closing in on your secret, a high-information guess can be better than a cautious guess - sometimes you just need to finish first.

Origins & History

Number Baseball belongs to the same deduction family as Bulls and Cows, an older paper-and-pencil code game. Wikipedia notes that Bulls and Cows predates Mastermind and that early computer versions such as Cambridge's MOO were already circulating around 1970. Mastermind then turned the same hidden-code idea into a commercial hit after Mordechai Meirovitz developed it in 1970 and Invicta released it in 1971-72; a later VICE retrospective described how that boom carried the format far beyond notebooks and classrooms.

Timeline

1968 Frank King created MOO at Cambridge as a computer version of the childhood puzzle later recognized as Bulls and Cows.
1970 Early computer play helped fix the game's modern shape, and Mordechai Meirovitz developed Mastermind from the same idea.
1971 Invicta began releasing Mastermind, turning the hidden-code format into an international board-game product.
1979 Mastermind championships and mass-market success showed how widely the Bulls-and-Cows style had spread in the 1970s.

Notable People

Frank King Created MOO, one of the earliest known computer versions of Bulls and Cows at Cambridge.
Mordechai Meirovitz Invented Mastermind in 1970 by adapting the older Bulls and Cows deduction idea.

Trivia

Wikipedia notes that the classic no-repeat 4-digit version can always be solved within seven turns.
The old Cambridge MOO program became so popular that administrators had to stop it from clogging the shared system.
Mastermind changed the clues from digits to colored pegs, but the core logic stayed close to Bulls and Cows.

Sources

FAQ

Is this the usual one-secret Number Baseball?

No. This build is a duel. You set your own secret first, the computer hides a different secret, and the first side to reach the full strike count wins.

Which modes are implemented here?

VS Computer Easy is 3 digits with 10 tries, VS Computer Normal is 3 digits with 9 tries, VS Computer Hard is 4 digits with 10 tries, and there are also Computer vs Computer Showmatch and Computer vs Computer Pro spectator modes.

Can digits repeat or start with 0?

No. Every secret and every guess must use unique digits, and the first digit is never allowed to be 0.

What does OUT mean on this page?

OUT means 0 strikes and 0 balls. In other words, none of the guessed digits appears anywhere in the target secret.

Why can I see the computer's history too?

Because this version is a race, not a solo puzzle. The second history panel lets you judge how quickly the computer is narrowing down your own secret.

Can I play with keyboard only?

Not at the moment. On desktop, use the on-screen keypad and buttons such as Recommend, Clear, Start Match, and Submit Guess.