Last Possible Number
Last Possible Number flips the question around: instead of asking which cell a digit goes in, it asks which digit a cell can take. Pick an empty cell, cross off every digit that already appears in its row, column, and box. If only one digit survives that sweep, that's the answer for the cell.
Focus on one cell
Pick any empty cell. Look at the digits already in its row — those can't go here. Same for its column. Same for its box. Whatever digit from 1 to 9 isn't in any of those three groups is a candidate. If only one candidate is left, that's the last possible number for the cell. Some books call this "cross-hatching" because you're mentally crossing out digits across three intersecting houses.
The only candidate
This move does its best work early in a solve, especially on easy and medium puzzles. Cells near the center of the grid are prime targets — they sit at the intersection of a row, a column, and a box, so they tend to have a lot of digits already ruling things out. It also pairs well with pencil notes: once candidates are filled in, any cell showing a single mark is a Last Possible Number. Even when several candidates remain, narrowing them down sets you up for the harder techniques later.
Practice makes it fast
With practice, scanning a cell's three houses turns into a split-second habit. Strong solvers do it hundreds of times in a single puzzle without consciously counting — they just see the missing digit. To build that instinct, work through medium puzzles and resist the hint button for the first ten minutes. Try to fill everything you can with Last Possible Number and Last Free Cell before switching to notes. The pattern recognition comes on its own, and after a while hard puzzles start feeling manageable.