- Column 5: 1, 4, 5, and 8 fit only in the highlighted cells.
- So these cells are taken by 1, 4, 5, and 8, one each.
- So these cells can hold no other numbers.
Hidden Quad in sudoku
A Hidden Quad is four numbers that, within one row, column or box, are noted only in the same four cells. Those cells must take the four numbers, so every other note in them can be removed. It is rare: unless all nine cells of the unit are empty, its other empty cells form a Naked Pair, Triple or Quad, which is easier to see.
Or scan with your phone
How to spot it
Count, for each number, the cells of a unit where it is noted, and look for four numbers confined to the same four cells. A row, column or box with no number in it yet is the place where nothing smaller can stand in for it.
Why it works
Four numbers need four cells of the unit, and only those four are open to them. No other number can take one of those cells.
On a real puzzle
This is Hidden Quad on its practice puzzle, at the first step that needs it. The game's hint explains the step like this:
Practice it
This Extreme puzzle calls for Hidden Quad. Fill in the singles first. When no number is obvious any more, look for the pattern, or ask for the hint: each game has one.