Navigating Medium Sudoku: The Bridge to Advanced Play

You've mastered the basics of easy Sudoku. Now, it's time to embrace a greater challenge. Medium Sudoku puzzles are where your true skills begin to form.

What Makes a Medium Sudoku Puzzle "Medium"?

The transition from easy to medium Sudoku is a significant step. While easy puzzles can often be solved by simply scanning for "naked singles," medium puzzles demand more sophisticated logic. They require you to not only see what *is* there but also to deduce what *must* be there based on what *isn't*.

Key Differences from Easy Puzzles

Medium puzzles typically have fewer "given" numbers, usually in the range of 25-30. This reduction in starting information means you'll have fewer obvious moves from the outset. The logical chains are longer, and you will absolutely need to use pencil marks (or the "Notes" feature) to keep track of potential candidates for each cell.

  • Fewer Given Numbers: This immediately increases the complexity, forcing you to rely on logic rather than simple observation.
  • Introduction of Intermediate Techniques: You can't rely solely on cross-hatching. You'll need to learn about concepts like "Hidden Singles" and "Locked Candidates."
  • Pencil Marks are Essential: Keeping track of candidate numbers for each empty cell becomes a core part of the solving process.
  • Requires More Patience: You will hit "walls" where no obvious move is available. This is normal. The solution lies in re-examining your notes and looking for more subtle patterns.

Core Intermediate Strategy: Pencil-Marking

If you haven't been using the notes feature, now is the time to start. The first step in tackling a medium puzzle should be to go through every empty cell and fill in all possible numbers (candidates) based on the initial givens. This process is called "Penciling In" or "Snyder Notation" (a more advanced version where you only note candidates if there are two or three in a box). For now, just fill in everything possible. This annotated grid is your new canvas.

The Power of Notes

Your pencil marks are not just a record; they are a tool. As you place correct numbers, you must diligently go back and erase those numbers from the candidate lists in the corresponding row, column, and box. This "clearing" of candidates is what reveals the next move. Learn more strategies on our Sudoku Tips page.

Essential Medium-Level Techniques

With your grid fully noted, you can now hunt for more advanced patterns. The most crucial technique at this level is finding "Hidden Singles."

Technique 1: Hidden Singles

A "Naked Single" is when an empty cell has only one possible candidate. A "Hidden Single" is more subtle. It's a situation where, within a specific row, column, or 3x3 box, a candidate number appears only once, even if the cell itself has other candidates.

  • How to Find It: Focus on a single unit (a row, column, or 3x3 box). Look at your pencil marks.
  • Scan through the numbers 1-9. Ask yourself: "In this box, where can the number '4' go?"
  • If you find that '4' is only listed as a candidate in *one single cell* within that box, then that cell *must* be a 4.
  • It doesn't matter if that cell also has '6' and '8' as candidates. If it's the only place for the '4' in that unit, you have found your answer.

Mastering the art of spotting Hidden Singles is the single most important skill for graduating from easy to medium puzzles.

Technique 2: Locked Candidates (Pointing and Claiming)

This technique involves finding a number that is a candidate in only one row or column within a 3x3 box. This allows you to eliminate that number as a candidate from other cells in that row or column outside the box.

  • Pointing Pairs/Triples: Look inside a 3x3 box. If the only candidates for a certain number (e.g., '7') are all in the same row, you have a "Pointing Pair" (or triple). Since one of those cells must be the '7' for that box, you can confidently eliminate '7' as a candidate from all other cells in that row, outside of that box.
  • Claiming Pairs/Triples: This is the reverse. Look at a row or column. If the only candidates for a certain number are all contained within a single 3x3 box, you can eliminate that number from all other cells within that box.

These "Locked Candidates" techniques are powerful because they don't solve a cell directly but clean up your pencil marks, which in turn will reveal new Naked or Hidden Singles.

Thinking in Chains of Logic

The essence of medium-level play is cause and effect. Placing a number in one cell has ripple effects across the grid. Your job is to follow these ripples. "If this cell is a 5, then that cell can no longer be a 5, which means..." This is the kind of logical flow you need to cultivate.

The Solving Process for a Medium Puzzle

  1. Initial Scan: First, solve all the obvious "Naked Singles" just as you would in an easy puzzle.
  2. Full Pencil-Marking: Once the easy moves are exhausted, fill in every possible candidate for every remaining empty cell.
  3. Hunt for Hidden Singles: Methodically scan each row, column, and box for Hidden Singles. This will likely be your most fruitful technique. When you find one, place the number.
  4. Update Pencil Marks: Immediately after placing a number, remove it as a candidate from all affected cells (the same row, column, and box). This step is critical.
  5. Look for Locked Candidates: If you get stuck, start looking for Pointing or Claiming patterns. Use them to eliminate candidates.
  6. Rinse and Repeat: The process is a loop. Placing a number or eliminating a candidate creates new information. Go back to step 3 and hunt for singles again. Continue this cycle until the puzzle is solved.

Ready for the Next Level?

Medium Sudoku puzzles are incredibly rewarding. They are the perfect balance of challenge and solvability, pushing your logical skills without being overwhelmingly difficult. They train your brain to see more complex patterns and build the foundation you'll need for hard and expert-level grids.