Master sudoku

Master sudoku, the hardest level

Master grids are the hardest on the site. They are rated by the hardest step they need, and that step is a chain: a line of linked candidates where each one being true or false forces the next. Many players never need them; some love them for exactly that reason.

What it takes

• Simple coloring
• X-chain
• XY-chain
• Alternating chain

Simple coloring follows one digit through the units where it has only two places, coloring the cells in two alternating colours. An X-chain does the same for one digit, an XY-chain links cells with two candidates each, and an alternating chain mixes both. They all end on one idea: one end or the other is true, so anything that sees both ends is ruled out.

And everything from Easy, Medium, Hard, and Expert.

How long it takes

An hour or more is normal. Take breaks: the clock pauses when you do.

Tips

• Take the grid as far as every simpler technique goes first: chains are easier to see with fewer candidates.
• Follow one digit at a time, through the rows, columns and boxes where it has only two places.
• The hint button explains each chain step by step, on the grid.