Thermo Sudoku Rules (Rainbow Bridge Mode)

What is Thermo Sudoku?

Thermo Sudoku is a Sudoku with thermometer shapes drawn on the grid. Along each thermometer, the numbers must strictly increase from the bulb to the tip. All the classic rules for rows, columns and boxes still apply.

In Marble Sudoku this mode is called Rainbow Bridge. Each bridge starts at the hole with a ring, and along it the marbles follow the tray order from left to right: Orange, Red, Pink, Teal, Green, Yellow, Purple, Navy, Sky. Colours can be skipped, but never go back.

9×9 board
Classic rules
Rising bridges

The rules

1
Classic rules. Each row, column and 3×3 box holds every number once.
2
Bridges rise. From the ringed start, each number on a bridge is higher than the one before it.
3
Skipping is fine. A bridge does not need every number. 1, 3, 4, 8 is allowed. 1, 3, 2 is not.
4
Nine in a row. A bridge of nine holes must hold 1 to 9 in order.

Watch it on the board

Each bridge starts at the hole with a ring.
Along a bridge, marbles follow the tray order from left to right. This bridge has 9 holes, so it holds all nine in order.
Shorter bridges can skip colours. Each marble just has to come later in the tray than the one before.
Fill a whole bridge to make it glow, then solve the rest of the board.

Tips

Count the length. On a bridge of length 5, the start can be at most 5 and the tip at least 5. Long bridges fix numbers fast.
Low at the start, high at the end. Write the smallest and largest possible number for each hole along a bridge, and remove the rest.
Watch where bridges cross lines. A bridge that runs inside one box or row also limits which numbers the other cells there can take.
Fill a bridge for bonus points. In Marble Sudoku a full bridge glows and adds bonus points, so finishing bridges first pays off.

Thermo Sudoku vs Classic Sudoku

	Thermo (Rainbow Bridge)	Classic
Rules	Classic rules plus rising bridges	Rows, columns, boxes
Givens	Often fewer, the bridges give extra clues	Given numbers only
Key skill	Counting the room along a bridge	Scanning and candidates

Thermo Sudoku FAQ

What are the rules of Thermo Sudoku?

Normal Sudoku rules apply, and along each thermometer the numbers must strictly increase from the bulb to the tip.

Can numbers repeat on a thermometer?

No. They must strictly increase, so no number can repeat and none can go down. Gaps are allowed.

What is Rainbow Bridge in Marble Sudoku?

It is the Thermo mode. Bridges start at a ringed hole, and marbles along a bridge follow the tray order from left to right.

What happens when a bridge is full?

In Marble Sudoku a completed bridge glows and earns bonus points.

Is Thermo Sudoku harder?

It adds a rule, but the bridges also give clues. Many players find it easier once they start counting along the bridges.

By the Luma Play team, makers of Marble Sudoku · Updated 27 October 2026