Sudoku
Strategies
Utilnivo grades puzzles by the techniques required on a human-style solving path — not by clue count alone.
Naked single
A cell has only one remaining candidate. Place it immediately.
Hidden single
A digit can appear in only one cell within a row, column, or box.
Pointing / claiming
When a digit in a box is confined to one row or column, eliminate it from the rest of that line.
Naked pair
Two cells in a unit share the same two candidates — remove those digits elsewhere in the unit.
X-Wing
A digit forms a rectangle across two rows and two columns, allowing eliminations on those columns (or rows).