What Is Rule 2 in Sudoku?

In most Sudoku guides, Rule 2 refers to the column rule: each column must contain the digits 1 through 9 exactly once. It is one of the three core rules that define a valid Sudoku puzzle.

This article explains Rule 2 clearly, shows how it affects solving, and provides practical examples and scan routines you can use immediately.

Rule 2, clearly stated

Each column must contain the digits 1, 2, 3, 4, 5, 6, 7, 8, and 9 exactly once.

No digit can repeat in a column, and every digit must appear.

How Rule 2 works with the other rules

Sudoku is built on three core rules:

1. Each row contains digits 1-9 once.
2. Each column contains digits 1-9 once. (Rule 2)
3. Each 3x3 box contains digits 1-9 once.

Rule 2 is not stronger or weaker than the other two. It simply adds another set of constraints that every placement must satisfy.

Example: using Rule 2 to place a digit

Suppose a column already contains 1, 2, 3, 4, 6, 7, and 9. The missing digits are 5 and 8.

If one of the two empty cells cannot be 8 because of its row or box, then it must be 5. This deduction is a direct use of Rule 2.

Example: finding a hidden single in a column

A hidden single is a digit that can only fit in one cell within a column. To find one:

1. List the missing digits in the column.
2. Check each empty cell’s row and box for conflicts.
3. If only one cell can take a digit, place it.

This is one of the fastest ways to progress in easy and medium puzzles.

Why Rule 2 is powerful

Rule 2 forces you to:

• Scan vertically as well as horizontally.
• Think about how row and box constraints interact with columns.
• Use elimination to reduce candidates.

Many hidden singles appear because a column has only one valid cell for a digit.

A simple column scan routine

Use this routine to apply Rule 2 efficiently:

• Scan a column and list missing digits.
• Check each empty cell for conflicts in its row and box.
• Place any forced digits immediately.
• Move to the next column.

If you repeat this scan after every few placements, you will catch most easy deductions quickly.

Common mistakes

• Only scanning rows: beginners often forget columns.
• Ignoring box interactions: a column might allow a digit, but the box may forbid it.
• Skipping candidate updates: after placing a digit, recheck the column immediately.

Practice exercise

Take an easy puzzle and solve it using columns first. You will be surprised how many placements appear once you train your eye to use Rule 2 consistently.

To increase difficulty, repeat the same exercise with a medium puzzle and limit yourself to two scans per column.

FAQ

Q1: Is Rule 2 always the column rule?

Most guides define Rule 1 as row and Rule 2 as column. The numbering can vary, but the rule itself is the same.

Q2: Are columns more important than rows?

No. Rows and columns are equal; they simply offer different scan perspectives.

Q3: Can a digit repeat in a column in variants?

No. Most variants keep the column rule and add extra constraints.

Q4: How do I find hidden singles in a column?

List missing digits and check which ones can appear in only one cell.

Q5: Is Rule 2 enough to solve a puzzle?

It is one of the three core rules. You need all three to solve Sudoku fully.

Q6: Does Rule 2 apply to 4x4 or 6x6 Sudoku?

Yes. The rule applies to any grid size; the symbols just change accordingly.

Skill reinforcement

For What Is Rule 2 in Sudoku?, spend one session doing only this technique. Do not solve the entire puzzle; stop after you have applied the method twice and verified both results. This builds accuracy and recognition faster than a full solve.

One-pass checklist

• Identify the exact pattern or constraint
• Apply one elimination
• Verify row, column, and box impacts

Repeat once more on a second puzzle to lock in the habit.

Skill reinforcement

For What Is Rule 2 in Sudoku?, spend one session doing only this technique. Do not solve the entire puzzle; stop after you have applied the method twice and verified both results. This builds accuracy and recognition faster than a full solve.

One-pass checklist

• Identify the exact pattern or constraint
• Apply one elimination
• Verify row, column, and box impacts

Repeat once more on a second puzzle to lock in the habit.

Skill reinforcement

For What Is Rule 2 in Sudoku?, spend one session doing only this technique. Do not solve the entire puzzle; stop after you have applied the method twice and verified both results. This builds accuracy and recognition faster than a full solve.

One-pass checklist

• Identify the exact pattern or constraint
• Apply one elimination
• Verify row, column, and box impacts

Repeat once more on a second puzzle to lock in the habit.