Letter Sudoku, or Wordoku, is a variety of the classic puzzle in which nine different letters are used instead of the numbers from 1 to 9. Most often the letters of the Latin alphabet — from A to I — are used. Sometimes you’ll find an ABC Sudoku format, where the initial letters of the alphabet are used.
Solving a letter Sudoku is based on the same logical principles as numeric Sudoku, and no knowledge of words or languages is required.
Rules for playing letter Sudoku
The board looks like a regular Sudoku — it’s a 9×9 grid divided into nine 3×3 blocks. The goal is to fill the grid so that:
- each row contains all 9 letters without repetition,
- each column — as well,
- and each 3×3 block — contains the same 9 letters, with no repetitions.
Some letters will already be filled in — they’re given at the start to help you get going. However, the number of letters provided at the beginning depends on the difficulty level.
Basic ways to solve letter Sudoku
Finding obvious letters (single-possibility method)
This method is especially useful at the very beginning, when the board is almost empty. The idea is simple: if in a row, column, or 3×3 block there is only one empty cell left and all the other letters are already placed, you can determine without error which letter must go there.
For example, if a row already contains A, B, C, D, E, F, G, and H, it’s obvious that the last empty cell must be the letter I. This strategy works particularly well in the small 3×3 blocks; they are the easiest place to start because they more quickly provide clues for the rest of the grid.
Elimination method
The elimination method helps you figure out where a specific letter can go when there isn’t — or you don’t see — an obvious option. Look at where this letter already appears on the board in other rows, columns, or blocks. In this way you can rule out the cells where it definitely cannot be.
To avoid getting confused, it’s best to make pencil marks directly in the cells, writing the possible letters there and crossing out the extra ones as you go. In the end, a single correct option should remain.
The only possible cell
This method is used when, in a row, column, or block, a certain letter can stand in only one place. Even if a 3×3 block has several empty cells, sometimes only one of them can take a particular letter without breaking the rules.
For example, the letter G may fit only in one cell of a block, because in all the other cells of that block it is already ruled out due to constraints from other rows/columns.
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Cross-logic method (scanning by rows and columns)
The essence of the method is to look at the row and the column simultaneously to determine where to place the needed letter.
Suppose the letter D must be in the lower-left block, but it isn’t yet clear in which cell. If D is already present in row 8 and in the first column, then it’s clear that it cannot be in the cells at the intersections of those lines. That leaves a single place where it can go. This approach works especially well when a letter already appears in two of the three blocks horizontally or vertically — then it’s easy to find where it’s missing.
It’s simple: look at where the letter already stands and exclude those rows and columns.
Finding “naked singles”
Finding a “naked single” is when, after all checks, only one possible letter remains for a cell. We’ve already ruled out everything that definitely doesn’t fit, and as a result there is a single letter left that can be entered there.
For example, it was assumed that a cell could be B, D, or G, but after looking at other rows and columns we realize that B and D don’t fit — therefore only G remains. In that case, enter it immediately. This is a very reliable technique and often helps to break a deadlock.
Using pencil marks (draft notes)
Using pencil marks is very convenient, especially if the Sudoku is difficult. The idea is to write in every empty cell all the letters that could go there. Then, as new letters are found elsewhere, cross out the ones that no longer fit. Pencil marks help you see where only one possible letter remains — which means you know exactly what to place. This method greatly simplifies the game and reduces the risk of errors.
Substitution method (trial-and-error technique)
The substitution method should be used when no other approaches help. Find a cell where two letters remain possible and try one of them. If at some point an error appears or everything leads to a dead end, go back and try the second option. The key is to remember where you started the assumption so you don’t get confused and have to start over. This method is almost like an experiment, so it’s better to use it only as a last resort.
How to combine the methods
To truly learn to solve Sudoku well — especially letter Sudoku — you need to be able to combine different techniques. First use the simplest options — the letters that jump out immediately. Then eliminate the extras and look at the intersections of rows and columns. If it gets difficult — make notes right in the cells; this helps you stay organized.
And if nothing works at all, you can try placing one of the possible letters and see where that leads. But it’s better to leave this option for the very last resort.
Common mistakes and how to avoid them:
- Ignoring the blocks
Beginners often focus only on rows and columns, forgetting the 3×3 blocks. Yet that’s often where the solution is hidden.- Mixing up letters
Be careful not to confuse similar letters: for example, C and G, or I and J. Use a clear, readable font or add color highlights.- Guessing without logic
Sudoku is a logic game, and you shouldn’t guess letters. Make notes and proceed step by step.
Letter Sudoku is not just an interesting alternative to numeric Sudoku; it’s also a great way to train your logic and attention. If you’re just starting out, don’t worry — pick the simplest puzzles, use clear techniques, and don’t rush. The main thing is not to be afraid of making mistakes and to learn from them. Over time you’ll start solving the harder variants easily and with enjoyment.
