Mastering the Fundamentals | 15 Minute Read
A Naked Single occurs when a specific cell has eight of the nine possible digits already present in its intersecting row, column, and 3x3 block. This leaves only one 'candidate' remaining. While simple, missing these can halt progress on even Medium-level boards.
A Hidden Single is slightly more advanced. It occurs when a number, say '5', can only appear in one cell within a block, even if that cell currently has other pencil marks (like 3 or 7) as possibilities. Mastering the scan for Hidden Singles is the first step toward Expert play.