Hidden Singles Strategy

Hidden Singles is the second fundamental technique you should learn. While Naked Singles focuses on individual cells, Hidden Singles examines entire units (rows, columns, boxes) to find where specific numbers must go.

What is a Hidden Single?

A Hidden Single occurs when a number can only appear in one cell within a unit (row, column, or box), even though that cell might have multiple candidate numbers.

How to Find Hidden Singles

Instead of looking at what a cell can contain, ask where a specific number can go:

1. Choose a number (e.g., 5)
2. Pick a unit (row, column, or box) that's missing that number
3. Check each empty cell in that unit to see if the number could fit
4. If only one cell can contain the number, you've found a Hidden Single

Example

Consider Row 3, which is missing the number 7:

• Cell A can't be 7 (there's a 7 in its column)
• Cell B can't be 7 (there's a 7 in its box)
• Cell C can't be 7 (there's a 7 in its column)
• Cell D could be 7 (no conflicts)
• Cell E can't be 7 (there's a 7 in its box)

Since Cell D is the only place in Row 3 where 7 can go, Cell D must be 7, even if it could also potentially hold other numbers.

Systematic Approach

To find Hidden Singles efficiently:

1. Focus on numbers 1-9 one at a time: Start with 1, find all Hidden Singles for that number, then move to 2, etc.
2. Check boxes first: Boxes are often easier to analyze than rows and columns
3. Look for almost-complete units: A box missing only 2-3 numbers is more likely to yield Hidden Singles

Hidden Singles vs. Naked Singles

When to Use This Technique

Hidden Singles becomes essential starting with medium-difficulty puzzles. Once you've exhausted all Naked Singles, switch to scanning for Hidden Singles before moving to more advanced techniques.

Practice

• Medium puzzle - requires Hidden Singles
• Hard puzzle - combines Naked and Hidden Singles

← Back to all strategies