Naked Pairs

Outcome

You can recognize two cells that reserve exactly two digits, identify the shared unit, and remove those digits from every other valid target in that unit.

Intro copy

Naked Pairs introduce a different kind of certainty:

You may not know which cell gets which digit, but you can still know which two cells own those two digits.

That reservation is enough to simplify the rest of the unit.

Step 1 — Make the reservation visible

Display:

{3,9} {3,9}

Prompt:

How many cells?

Answer: 2.

How many distinct candidates across them?

Answer: 2.

UI shows:

2 cells / 2 digits → reserved

Then reveal other candidate 3s and 9s in the row.

Learner removes them.

Step 2 — Guided row Pair

Required state: Two {2,7} cells in one row, with multiple 2/7 eliminations elsewhere.

Prompt sequence:

1. identify pair cells;
2. identify pair digits;
3. identify shared unit;
4. remove every pair digit elsewhere in the unit.

Correct feedback:

Good. The pair cells must use 2 and 7 in some order, so the rest of the row cannot use either digit.

Step 3 — Box Pair

Prompt:

Same candidate relationship. Different unit. What changes?

Expected:

Eliminations happen elsewhere in the box.

Step 4 — False Pair

Show:

• {2,7}
• {2,8}

Prompt:

Is this a Naked Pair?

Correct: No.

UI combines candidates:

Union = {2,7,8}

Feedback:

Two cells but three digits. Nothing is fully reserved.

Step 5 — Pair in two units

Two {4,9} cells share both:

• a row;
• a box.

Prompt:

Where can the pair eliminate?

Correct:

• rest of row;
• rest of box.

Introduce the alias Locked Pair after the learner solves the logic.

Practice

States:

1. row;
2. column;
3. box;
4. two-unit Locked Pair;
5. false pair;
6. valid pair with no productive elimination — classify as true but currently unproductive.

Hint ladder

1. “Look for bivalue cells.”
2. “Compare candidate sets inside one unit.”
3. “These two cells share the same two candidates.”
4. “Those two digits are reserved here.”
5. Highlight elimination targets.

Checkpoint

No cells are highlighted.

Prompt:

Find one two-cell reservation and make every elimination it allows.

Learner must identify:

• pair cells;
• pair digits;
• unit(s);
• targets.

Completion

Four productive Pairs, one false-pair rejection, and one true-but-unproductive classification.

Next

Hidden Pairs

Now find the same two-for-two reservation by following digits rather than tidy cell candidate sets.

Reference

Read Naked Pair for Locked Pairs, the union test, and the full Naked-vs-Hidden comparison.