A Sudoku solution is a chain of small deductions. Every row, column
and three-by-three box contains the digits one to nine once each. The most satisfying move is the one you can explain
before you write it.
FORM STUDY / SUDOKUA STUDY IN FORM
Three things to learn
THE ESSENTIAL PRACTICE
◇
Build useful pencil marks
Scan the row, column and box around an empty square. Cross out every digit already present in any of those three
houses; the remaining digits are its candidates. Write small marks consistently so you can compare cells without
searching. Candidates are possibilities rather than commitments. After entering a confirmed digit, remove it from the
other cells in the same row, column and box. Work around that newly changed area first. Keeping marks up to date
prevents an old possibility from masquerading as a new discovery. If a cell loses every candidate, backtrack to your
last certain placement and check for a transcription mistake.
◇
Find two kinds of single
A naked single is a cell with only one candidate left. A hidden single is a digit that can fit in only one cell of
a row, column or box, even if that cell has several candidates. Try scanning for a particular digit across three
adjacent boxes, using existing placements to rule out rows or columns. Then change digits and scan again. Neither
technique requires a guess. Say the reason aloud: this is the only place for seven in this box. That sentence makes
your move easier to audit and helps you recognise the same pattern in a different part of the grid.
◇
Let pairs do the pruning
If two cells in one house have exactly the same two candidates, those digits must occupy those two cells in some
order. Remove both digits from all other cells in that house. The pair does not yet tell you which cell holds which
digit. A pair can belong to a row, a column or a box; check the shared house carefully before making eliminations.
Work in pencil until a cell becomes forced. When a puzzle seems stuck, review candidate accuracy and look for singles
created by your last elimination. A short break often reveals a house you have been overlooking.
THE WORKING DESK
A first grid to finish
Six cells are missing from this completed beginner board. Use
row, column and box exclusions to recover them. Check your entries at any time, or reveal one hint when you want a
nudge. Reset gives you the same six deductions to practise again.