Coding-Decoding Questions: Concepts, Types, Solved Examples and Practice
Coding-Decoding is an important topic in logical reasoning where letters, numbers, words, or symbols are transformed according to a specific rule.
In a coding-decoding question, you are usually given an original word, number, or sequence along with its coded form. Your task is to identify the rule used for the transformation and apply the same rule to another word or sequence.
For example:
CAT → DBU
Here:
- C becomes D
- A becomes B
- T becomes U
Each letter moves one position forward in the alphabet.
Using the same rule:
DOG → EPH
Coding-decoding questions may look confusing at first, but most of them follow a systematic pattern. Once you learn how to identify the transformation, these questions can become quick scoring opportunities in aptitude tests.
Quick answer
Coding-Decoding is a reasoning topic in which letters, numbers, words, or symbols are changed according to a hidden rule. You need to identify that rule and use it to find the correct code or decode the given information.
Common coding-decoding patterns include:
- Letter shifting
- Reverse alphabet coding
- Number-based coding
- Symbol and sign coding
- Word coding
- Substitution coding
- Position-based coding
- Rearrangement coding
- Matrix or grid coding
- Fictitious language coding
- Jumbled coded sentence coding
- Mixed coding
The most important skill is pattern recognition.
What is coding-decoding?
Coding means changing information from its original form into another form according to a particular rule.
Decoding means using that rule to recover or understand the original information.
Consider:
APPLE → BQQMF
Compare each letter:
- A → B
- P → Q
- P → Q
- L → M
- E → F
Each letter moves one position forward.
Therefore, the coding rule is:
Next letter = Current letter + 1
If the question asks you to code:
MANGO
then:
- M → N
- A → B
- N → O
- G → H
- O → P
So:
MANGO → NBOHP
The process is simple once the rule is identified.
Why is coding-decoding important?
Coding-decoding questions test your ability to identify relationships and apply rules consistently.
They help test:
- Logical thinking
- Pattern recognition
- Observation
- Alphabet knowledge
- Analytical ability
- Speed and accuracy
- Problem-solving skills
These questions are also useful because they usually do not require lengthy calculations.
Pocket Aptitude Tip: In coding-decoding, the hardest step is usually identifying the rule. Once the rule is clear, applying it is often straightforward.
Types of coding-decoding questions
Coding-decoding questions can be presented in different ways.
| Type | Basic idea |
|---|---|
| Letter Coding | Letters are shifted or transformed |
| Number Coding | Letters or words are represented using numbers |
| Symbol Coding | Letters, numbers, or operators are replaced with symbols |
| Substitution Coding | One word or symbol is replaced by another |
| Reverse Coding | Letters are coded using their reverse alphabet positions |
| Word Coding | Complete words are assigned codes |
| Rearrangement Coding | Letters are rearranged according to a rule |
| Matrix/Grid Coding | Letters are coded by their row and column position in a grid |
| Fictitious Language Coding | Coded sentences are compared to find the code for each word |
| Jumbled Coded Sentence | Same as fictitious language coding, but the coded word order does not match the sentence order |
| Mixed Coding | More than one rule is used |
Understanding these types helps you recognise the method faster during an examination.
Letter coding
Letter coding is one of the simplest forms of coding-decoding.
The letters in a word are changed according to a fixed rule.
Example
If:
BAT → CBU
Compare the letters:
- B → C
- A → B
- T → U
Each letter moves one position forward.
Therefore:
DOG → EPH
because:
- D → E
- O → P
- G → H
Another example
Suppose:
CAT → BZS
Here:
- C → B
- A → Z
- T → S
Each letter moves one position backward.
Therefore:
DOG → CNF
because:
- D → C
- O → N
- G → F
The important point is to identify whether the letters are moving forward or backward.
Alphabet positions
Knowing alphabet positions makes many coding-decoding questions easier.
| Letter | Position | Letter | Position |
|---|---|---|---|
| A | 1 | N | 14 |
| B | 2 | O | 15 |
| C | 3 | P | 16 |
| D | 4 | Q | 17 |
| E | 5 | R | 18 |
| F | 6 | S | 19 |
| G | 7 | T | 20 |
| H | 8 | U | 21 |
| I | 9 | V | 22 |
| J | 10 | W | 23 |
| K | 11 | X | 24 |
| L | 12 | Y | 25 |
| M | 13 | Z | 26 |
For example:
C = 3
M = 13
T = 20
Z = 26
You do not always need to memorise all 26 positions immediately. With regular practice, the most commonly used positions become easier to recall.
Forward letter shifting
In forward shifting, each letter moves a fixed number of positions ahead.
Suppose:
CAT → FDW
Check the transformation:
- C → F = +3
- A → D = +3
- T → W = +3
The rule is:
Move every letter 3 positions forward.
Now apply the same rule to:
DOG
- D → G
- O → R
- G → J
Therefore:
DOG → GRJ
Important point about Z
When shifting letters forward, the alphabet can wrap around.
For example, if a letter moves one position forward:
Z → A
Similarly:
Y → Z
This is important when a coding rule involves letters near the end of the alphabet.
Backward letter shifting
In backward shifting, each letter moves a fixed number of positions backward.
Suppose:
DOG → ALD
Check:
- D → A = −3
- O → L = −3
- G → D = −3
Therefore, the rule is:
Move every letter 3 positions backward.
Now apply it to:
CAT
- C → Z
- A → X
- T → Q
Therefore:
CAT → ZXQ
Reverse alphabet coding
Another common method uses the opposite letter positions in the alphabet.
The basic pairs are:
- A ↔ Z
- B ↔ Y
- C ↔ X
- D ↔ W
- E ↔ V
- F ↔ U
- G ↔ T
- H ↔ S
- I ↔ R
- J ↔ Q
- K ↔ P
- L ↔ O
- M ↔ N
For example:
A → Z
B → Y
C → X
D → W
Consider:
CAT
Using reverse alphabet coding:
- C → X
- A → Z
- T → G
Therefore:
CAT → XZG
This type of coding can be solved quickly once the reverse alphabet pairs are familiar.
Speed Improvement Tip: Memorise the pairs A-Z, B-Y, C-X, D-W, E-V and so on. This is faster than calculating the reverse position separately for every letter.
Number coding
In number coding, letters may be represented using their alphabet positions.
For example:
A = 1
B = 2
C = 3
and so on.
Therefore:
CAT
becomes:
3 - 1 - 20
because:
- C = 3
- A = 1
- T = 20
Similarly:
DOG
becomes:
4 - 15 - 7
because:
- D = 4
- O = 15
- G = 7
However, not every number-coding question uses direct alphabet positions.
The question may introduce another mathematical relationship.
Therefore, always compare the examples given in the question before assuming the coding rule.
Word coding
In word coding questions, complete words are assigned codes.
For example:
If:
APPLE = 25
and:
MANGO = 30
the question may ask you to identify the code for another word.
The code could be based on:
- Number of letters
- Alphabet positions
- Sum of letter values
- Product of selected values
- Number of vowels
- Number of consonants
- A combination of rules
You should not assume the rule without checking the information provided.
Example
Suppose the code of a word is the sum of the alphabet positions of its letters.
For:
BAD
B = 2
A = 1
D = 4
Therefore:
2 + 1 + 4 = 7
So:
BAD → 7
Now consider:
CAT
C = 3
A = 1
T = 20
Therefore:
3 + 1 + 20 = 24
So:
CAT → 24
Substitution coding
In substitution coding, one word may be represented by another word.
For example:
If:
“book” is called “pen”
and
“pen” is called “bag”
then the question may ask what is used for writing.
The actual object used for writing is a pen, but according to the coding system, pen is called bag.
Therefore, the answer is bag.
Common Mistake: Do not answer using the normal meaning of the word. Follow the naming rule given in the question.
Rearrangement coding
Some coding questions change the order of letters instead of simply shifting them.
For example:
TABLE → ELBAT
The letters have been reversed.
Therefore:
CHAIR → RIAHC
If the question provides a more complicated rearrangement, compare the original and coded words letter by letter to identify the position changes.
Symbol and sign coding
In symbol coding, letters, numbers, or even mathematical operators are replaced with symbols such as @, #, %, and & instead of other letters.
Example
If:
A = @, B = #, C = %, D = &
then:
BAD → #@&
because:
- B → #
- A → @
- D → &
Operator substitution
Sometimes the symbols for mathematical operators are swapped instead.
Suppose:
’+’ means ’×’, ’×’ means ’÷’, ’−’ means ’+’
Evaluate:
10 + 2 × 5 − 3
Replace each symbol with its new meaning first:
10 × 2 ÷ 5 + 3
Then solve normally:
20 ÷ 5 + 3 = 4 + 3 = 7
Speed Improvement Tip: When operators are swapped, rewrite the full expression with the new symbols before calculating anything. Solving as you substitute leads to mistakes.
Matrix or grid coding
In matrix coding, letters are placed inside a grid, and each letter’s code is written as its row and column number.
A common 5×5 grid (I and J share a cell) looks like this:
| 1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
| 1 | A | B | C | D | E |
| 2 | F | G | H | I | K |
| 3 | L | M | N | O | P |
| 4 | Q | R | S | T | U |
| 5 | V | W | X | Y | Z |
Example
Using row-then-column order, code CAT.
- C is in row 1, column 3 → 13
- A is in row 1, column 1 → 11
- T is in row 4, column 4 → 44
Therefore:
CAT → 13-11-44
Decoding example
Decode 43-45-33 using the same grid.
- 43 → row 4, column 3 → S
- 45 → row 4, column 5 → U
- 33 → row 3, column 3 → N
Therefore:
43-45-33 → SUN
Some questions reverse the order to column-then-row, so always check which order the question uses before solving.
Fictitious language coding
In this type, complete sentences are given in an invented language along with their meaning. There is no shifting or symbol rule to find — instead, you compare two sentences that share exactly one common word to identify exactly one matching code word.
Example
If:
“pit na ta” means “bring red rose”
“ta ki pa” means “rose is fresh”
find the code for rose.
Both sentences share the word “rose” and the code word “ta”. Since this is the only word and only code they have in common:
ta = rose
Extending with a third sentence
If:
“mo di sil” means “cats eat fish”
“sil ta re” means “fish is tasty”
“re ko mo” means “tasty and cats”
compare sentences two at a time:
- Sentences 1 and 2 share “fish” and “sil” → sil = fish
- Sentences 2 and 3 share “tasty” and “re” → re = tasty
- Sentences 1 and 3 share “cats” and “mo” → mo = cats
- The leftover word in sentence 3 is “and”, and the leftover code is “ko” → ko = and
Common Mistake: Never assign a code when two sentences share more than one word and more than one code word at the same time. That comparison is ambiguous — wait until you have a sentence that shares exactly one word with exactly one code.
Jumbled coded sentence coding
This works the same way as fictitious language coding, except the order of the coded words does not match the order of the real sentence. Because position cannot be trusted, you must rely only on comparisons where exactly one word and one code overlap.
Example
If:
“de fa go” means “sun is bright”
“go ha” means “bright day”
“de ka la” means “sun rises early”
find the codes for sun, bright, is, and day.
- Sentences 1 and 2 share only “bright” and “go” → go = bright
- Sentences 1 and 3 share only “sun” and “de” → de = sun
- In sentence 1, the leftover word is “is” and the leftover code is “fa” → fa = is
- In sentence 2, the leftover word is “day” and the leftover code is “ha” → ha = day
Mixed coding
Some questions use more than one operation.
For example:
CAT → DBU
may use:
- Letter shift +1
Another question could use:
- Reverse the word
- Then shift each letter
- Then assign numbers
When multiple operations are involved, solve them one step at a time.
Do not try to identify the entire rule mentally in one step.
How to solve coding-decoding questions
Use the following method whenever you encounter a new coding-decoding question.
Step 1: Write the original information
Write the given word, number, or sequence clearly.
Step 2: Write the coded information
Place the code directly below it.
For example:
C A T
D B U
Step 3: Compare corresponding positions
Check:
- C → D
- A → B
- T → U
Step 4: Look for a common rule
Ask:
- Are the letters moving forward?
- Are they moving backward?
- Are they being reversed?
- Are they being replaced?
- Are alphabet positions being used?
- Is there a mathematical operation?
- Are multiple operations involved?
Step 5: Test the rule
Do not stop after finding a pattern in only one letter.
Check the rule against every available letter or example.
Step 6: Apply the rule
Once the rule works consistently, apply it to the word or number asked in the question.
Step 7: Verify
Check the final answer one more time.
This method reduces errors and prevents guessing.
Solved example 1: Letter shifting
Question:
If MANGO is coded as NBOHP, how is APPLE coded using the same rule?
Solution
Compare:
M → N
A → B
N → O
G → H
O → P
Each letter moves one position forward.
Now apply the same rule to APPLE:
- A → B
- P → Q
- P → Q
- L → M
- E → F
Therefore:
APPLE → BQQMF
Answer: BQQMF
Solved example 2: Backward shifting
Question:
If DOG is coded as ALD, how is CAT coded?
Solution
D → A
O → L
G → D
Each letter moves 3 positions backward.
Now:
C → Z
A → X
T → Q
Therefore:
CAT → ZXQ
Answer: ZXQ
Solved example 3: Reverse alphabet
Question:
If A is coded as Z, B as Y, and C as X, how is DOG coded?
Solution
Use reverse alphabet positions:
- D → W
- O → L
- G → T
Therefore:
DOG → WLT
Answer: WLT
Solved example 4: Alphabet position coding
Question:
If each letter is represented by its alphabet position, what is the code for BAD?
Solution
- B = 2
- A = 1
- D = 4
Therefore:
BAD → 2-1-4
Answer: 2-1-4
Solved example 5: Word value
Question:
If the code of a word is the sum of the alphabet positions of its letters, find the code for DOG.
Solution
D = 4
O = 15
G = 7
Therefore:
4 + 15 + 7 = 26
Answer: 26
Solved example 6: Symbol coding
Question:
If A = @, B = #, C = %, D = &, what is the code for BAD?
Solution
- B → #
- A → @
- D → &
Therefore:
BAD → #@&
Answer: #@&
Solved example 7: Matrix coding
Question:
Using the 5×5 grid above (row, then column), what is the code for CAT?
Solution
- C is in row 1, column 3 → 13
- A is in row 1, column 1 → 11
- T is in row 4, column 4 → 44
Therefore:
CAT → 13-11-44
Answer: 13-11-44
Solved example 8: Fictitious language coding
Question:
If “pit na ta” means “bring red rose” and “ta ki pa” means “rose is fresh”, what is the code for rose?
Solution
The two sentences share exactly one word, “rose”, and exactly one code word, “ta”.
Therefore:
rose → ta
Answer: ta
Solved example 9: Jumbled coded sentence
Question:
If “de fa go” means “sun is bright” and “go ha” means “bright day”, what is the code for day?
Solution
The two sentences share exactly one word, “bright”, and exactly one code word, “go”, so go = bright.
In the second sentence, the only word left is “day” and the only code left is “ha”.
Therefore:
day → ha
Answer: ha
Common mistakes in coding-decoding
1. Finding a pattern from only one letter
A transformation may appear correct for the first letter but fail for the remaining letters.
Better approach: Always test the rule against the complete example.
2. Forgetting alphabet wrapping
Students sometimes assume that Z has no next letter.
But in many coding questions:
Z → A
and:
A → Z
depending on the direction of the shift.
3. Confusing forward and backward shifts
If C becomes F, the shift is +3.
If F becomes C, the shift is −3.
Write the positions if you are unsure.
4. Assuming direct alphabet positions
Not every number code means A = 1, B = 2, C = 3.
The question may use a different rule.
Better approach: Identify the rule from the examples before solving.
5. Ignoring word order
Some questions reverse or rearrange letters.
If you only compare the first and last letters, you may miss the actual pattern.
6. Overcomplicating a simple pattern
Students sometimes search for complicated mathematical rules when the letters are simply shifted by one or two positions.
Better approach: Start with simple patterns before considering complicated ones.
Fast approach for coding-decoding
During an examination, use this order:
- Check for direct letter shifting.
- Check for backward shifting.
- Check for reverse alphabet coding.
- Check for letter rearrangement.
- Check alphabet positions.
- Check numerical operations.
- Check whether more than one rule is being used.
This gives you a systematic search order instead of random guessing.
How to improve coding-decoding speed
Speed improves mainly through familiarity.
Learn common alphabet positions
Start by becoming comfortable with:
A = 1, E = 5, I = 9, M = 13, N = 14, O = 15, T = 20, Z = 26
Then gradually learn all alphabet positions.
Practise small shifts
Practise:
- +1
- +2
- +3
- −1
- −2
- −3
Once these become familiar, many basic questions can be solved mentally.
Practise reverse alphabet pairs
Remember:
A-Z
B-Y
C-X
D-W
E-V
F-U
This helps with reverse coding questions.
Use a written comparison
If you are unsure, write:
A B C D
D E F G
This makes the transformation visible and reduces mental errors.
Practice questions
Question 1
If CAT is coded as DBU, how is DOG coded?
A. EPH
B. EOG
C. FPH
D. DNF
Answer: A. EPH
Question 2
If each letter is moved two positions forward, how is BAT coded?
A. DCV
B. DCU
C. BCV
D. ZYR
Answer: A. DCV
Question 3
If each letter is moved one position backward, how is DOG coded?
A. CNF
B. EPH
C. CNE
D. BME
Answer: A. CNF
Question 4
Using reverse alphabet coding, what is the code for CAT?
A. XZG
B. YAG
C. XZH
D. WZG
Answer: A. XZG
Question 5
If A = 1, B = 2, C = 3 and so on, what is the code for DOG?
A. 4-15-7
B. 5-14-8
C. 4-14-7
D. 3-15-8
Answer: A. 4-15-7
Question 6
If the code of a word is the sum of its alphabet positions, what is the code for CAT?
A. 22
B. 23
C. 24
D. 25
Answer: C. 24
Question 7
If APPLE → BQQMF, what is the coding rule?
A. Each letter moves one position forward
B. Each letter moves one position backward
C. Letters are reversed
D. Letters are replaced randomly
Answer: A. Each letter moves one position forward
Question 8
If MANGO → NBOHP, how is GRAPE coded using the same rule?
A. HSBQF
B. HSBPF
C. HRAQF
D. GSBQF
Answer: A. HSBQF
Question 9
If A = @, B = #, C = %, D = &, what is the code for CAB?
A. %@#
B. %#@
C. @#%
D. #@%
Answer: A. %@#
Question 10
Using the 5×5 grid (row, then column) shown above, what is the code for DOG?
A. 14-43-22
B. 41-34-22
C. 14-34-22
D. 41-43-22
Answer: A. 14-43-22
Question 11
If “mo di sil” means “cats eat fish” and “sil ta re” means “fish is tasty”, what is the code for fish?
A. mo
B. di
C. sil
D. ta
Answer: C. sil
Question 12
If “de fa go” means “sun is bright” and “de ka la” means “sun rises early”, what is the code for sun?
A. fa
B. go
C. ka
D. de
Answer: D. de
Coding-decoding revision table
| Pattern | What to check |
|---|---|
| Forward shift | Letters move ahead by a fixed number |
| Backward shift | Letters move back by a fixed number |
| Reverse alphabet | A-Z, B-Y, C-X pattern |
| Alphabet position | A=1, B=2, …, Z=26 |
| Symbol coding | Letters, numbers, or operators replaced by symbols |
| Word value | Sum or operation using letter positions |
| Rearrangement | Letters change their positions |
| Matrix/grid coding | Code is a letter’s row and column position |
| Fictitious language | Compare sentences to find one word’s code |
| Jumbled coded sentence | Same as above, but code word order does not match sentence order |
| Substitution | One word or object is given another name |
| Mixed coding | Two or more rules are combined |
Pocket Aptitude tip
When you see a coding-decoding question, do not immediately start calculating.
First ask:
“What changed between the original and coded form?”
Then check the simplest possibilities:
Shift → Reverse → Position → Rearrangement → Mixed rule
This small habit can save time and reduce unnecessary calculations.
Frequently asked questions
What is coding-decoding in logical reasoning?
Coding-decoding is a reasoning topic where letters, numbers, words, or symbols are transformed according to a specific rule. You must identify the rule and apply it to find the required code or original information.
How do I solve coding-decoding questions?
First compare the original and coded information. Check for letter shifts, reverse alphabet positions, numerical values, rearrangement, substitution, or a combination of rules.
Do I need to memorise alphabet positions?
Knowing alphabet positions makes coding-decoding faster, especially for number-based questions. However, you can learn them gradually through regular practice.
What are the most common types of coding-decoding?
Common types include letter shifting, backward shifting, reverse alphabet coding, number coding, symbol coding, word coding, substitution coding, rearrangement coding, matrix/grid coding, fictitious language coding, jumbled coded sentence coding, and mixed coding.
How can I solve coding-decoding questions faster?
Start by checking simple patterns such as forward or backward letter shifts. With practice, you will recognise common transformations quickly and spend less time testing unnecessary possibilities.
Is coding-decoding important for placement aptitude tests?
Coding-decoding is an important logical reasoning topic to practise for aptitude assessments. The exact topics and question patterns vary between examinations and assessments.
Related articles
Coding-decoding should be connected with the other reasoning resources in the Pocket Aptitude topic cluster:
- Number Series
- Blood Relations
- Seating Arrangement
- Directions
- Logical Reasoning Questions
- Logical Reasoning Practice Questions
Conclusion
Coding-decoding questions become easier when you stop treating them as random puzzles and start looking for the transformation rule.
Begin with simple letter shifts, then learn reverse alphabet coding, alphabet positions, number coding, word coding, substitution, and rearrangement patterns. Once you are comfortable with these patterns, practise mixed questions under a time limit.
The most important habit is simple:
Compare → Identify the rule → Apply the rule → Verify the answer
With consistent practice, coding-decoding can become one of the faster logical reasoning topics to solve in an aptitude test.