Grafite Sudoku
Locked candidates in sudoku
Locked candidates (in two forms, often called pointing and claiming) are the first technique you need when the singles run out. They work on pencil marks: the possible numbers written small in each cell.
From box to row (pointing)
If, inside a box, a number can only sit in cells of the same row, then in that row the number must be inside that box. You can remove it from the pencil marks of the other cells of the row, outside the box.
In the example, in the top-left box the 4 can only go in row 2. So in row 2 the 4 is "locked" into that box, and it is removed from the pencil marks in column 5 and column 8.
From row to box (claiming)
It also works the other way round: if in a row a number can only sit in cells of the same box, remove it from the other cells of that box. The same reasoning works with columns.
How to spot it
Write the pencil marks of a box and look for a number that appears in only one row or one column. You really need pencil marks for this one: without them it almost always slips by.
In the app: "Locked candidate: box to row" lesson in the Sudoku Academy
In the Sudoku Academy of Grafite Sudoku this technique is an interactive lesson on the grid, with exercises from real puzzles.