Naked Singles
A Naked Single is what you get when a cell's candidates — every digit not yet ruled out by its row, column, and box — shrink down to just one. That lone candidate is the naked single: the only digit that can legally go in the cell. Spotting one is one of the more satisfying moments in a solve, because it's a placement that's fully nailed down by logic.
Spotting naked singles
Naked singles only show up once you've filled in pencil marks across the board. Scan systematically — cell by cell, row by row. Any cell with a single candidate is a naked single, and you can place that digit with full confidence. On Sudoked.com, the hint feature will point them out when you're stuck and walk you through why the highlighted cell has to take that digit. Building the habit of scanning your notes for lone candidates is the single biggest thing you can do to speed up your solving.
The simplest advanced move
Every time you place a naked single, you strip that digit from the candidate lists of every other cell in the same row, column, and box. That cascade often produces new naked singles in neighboring cells. A typical medium or hard solve plays out as a chain reaction: spot one, place it, update the notes, and the next one is already staring at you. That cascade is what smooth solving feels like. Strong players learn to see the chain coming and plan a few placements ahead.
Build the chain
It's worth knowing the difference between naked and hidden singles, even though both end in a placement. A naked single is right there in the cell's notes — you see one candidate, period. A hidden single is a digit that can only fit in one cell of a house, but that cell also holds other candidates — so the digit is buried among them. Naked singles are easier to spot once notes are filled, which is why beginners get told to master notes and naked singles before moving on to hidden singles, pairs, triplets, and the rest of the technique family.