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ELI5 Yahaaa

A set of heights, not a shape

Why Does Only One Key Fit?

Every notch on a key lifts one pin to exactly the height that frees the barrel.

A row of spring-loadedpinsEach cut to a differentheightThe key lifts every oneexactlyOnly then can thebarrel turn
A key is not a shape. It is a set of heights.

  1. Step 1 of 5

    Inside a lock is a row of little pins.

    A lock from the side Each pin cut into twopieces

    Springs push a column of small pins down into the rotating barrel, and each column is cut into two separate pieces at a chosen height.

  2. Step 2 of 5

    The split must line up with the barrel edge.

    Top pins pushed byspringsheld down intothe barrelThe split linemust sitexactly at theedgeBottom pins on the keylifted to theright height

    While any split sits above or below the edge of the barrel, a pin bridges the gap and physically stops the barrel from rotating.

  3. Step 3 of 5

    Every notch sets one pin's height.

    1 The key slidesin2 Each notchlifts its ownpin3 All the splitsline up at once

    As the key slides in, each notch meets its own pin and raises it by however much that particular cut happens to allow.

  4. Step 4 of 5

    One wrong height blocks everything.

    The right key All pins aligned — itturnsA nearly-right key?One pin still blocks it

    It does not matter how many pins are correct. A single pin left too high or too low still bridges the barrel and the key will not turn.

  5. Step 5 of 5

    The number of combinations is the security.

    One pin — a fewheightsTwo pins —heightsmultipliedSix pins —thousands ofkeys

    Each extra pin multiplies the possibilities rather than adding to them, which is how a handful of pins becomes thousands of different keys.

    A common six-pin lock with four usable heights per pin has about 4,000 combinations, far fewer than most people assume.

The short version

Only one key fits a lock because every notch must lift its own pin to exactly the right height before the barrel is free to turn.

Try it yourself

Look along the edge of your front door key. The notches are all different depths, and each one is a single pin's instruction.

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