Naked Singles
A Naked Single resolves the exact-one constraint in a cell. Its row, column, box, and earlier deductions have already excluded eight digits, leaving one candidate true. Resolving that candidate also excludes it from all 20 peers; writing it as a large solved digit is the compact record of those exclusions.
Count down to one
A complete candidate state must preserve every exclusion made by earlier sound deductions. If eight digits are false in a cell, the ninth is the Naked Single. Pencilmarks make that state visible. When all eight exclusions come directly from digits already placed in the cell's row, column, and box, the same survivor can be found without any notes — that scan is the Last Possible Number from the Basics. Otherwise the earlier eliminations must remain recorded in the candidate state.
Apply the peer exclusions
Resolving the surviving candidate makes the same digit false in every one of the cell's 20 peers. Remove each visible pencilmark for that digit from the shared row, column, and box. Writing the surviving candidate as a large solved digit is useful bookkeeping, but the logical effect is the set of peer exclusions it represents.
Naked vs hidden singles
Both Singles use exact-one constraints, but their premises face opposite directions. A Naked Single starts with eight digit exclusions inside one cell and then excludes the surviving digit from 20 peers. A Hidden Single starts with eight position exclusions for one digit inside a house, then excludes every other digit from the surviving cell and excludes the resolved digit from the 12 peers outside that source house.