Variant Sudoku

Thermo Sudoku Rules Explained in One Short Example

0:15 · Thermometers · Variant Sudoku

A four-cell thermometer shows the rule. 2, 5, 7, 3 fails because 7 drops to 3, and 2, 5, 7, 8 climbs all the way from the bulb to the tip.

Full lesson and practicePlay Thermo Sudoku

Transcript

  1. 0:02Digits rise from the bulb end of a thermometer.
  2. 0:067 then 3 goes down, so this breaks the thermometer.
  3. 0:092, 5, 7, 8 climbs all the way. Equal digits aren’t allowed either.

The rule

A thermometer is a thick line with a round bulb at one end. Digits along it must strictly increase as you move away from the bulb, so no two cells on a thermo are equal. Normal sudoku rules still apply.

Long thermometers squeeze the digits hard. A 9-cell thermo can only be 1 to 9 in order, and even a 4-cell thermo rules out several digits in every cell.

Worth memorizing

  1. On a thermo of n cells, the bulb is at most 10 − n and the tip is at least n.

    A 4-cell bulb is 1 to 6.

  2. Equal digits aren’t allowed unless the puzzle says “slow” thermo.

  3. A 9-cell thermo is exactly 1 to 9 in order.

Practice thermometers in the lesson

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