This page covers SSC GD Constable Coding Decoding with complete concept notes, 15 graded practice MCQs, key points and exam-specific tips. Free to study.
Core ConceptRead this first — the foundation of the topic
Core Concept
In coding, each letter or word is replaced by another letter, number, or symbol following a fixed rule. In decoding, you reverse this process to find the original word or its coded form
Letter Shift Coding
Each letter moves forward or backward by a fixed number in the alphabet.
2. Number/Position Coding: Letters are replaced by their position numbers in alphabet.
3
Substitution Coding
Specific letters are replaced by other specific letters.
4
Mixed Letter Coding
Combination of different coding methods
Formula for Letter Shift Coding
If A=1, B=2, C=3... Z=26
Forward Shift: New Position = (Original Position + Shift Value)
Backward Shift: New Position = (Original Position - Shift Value)
If result > 26, subtract 26. If result < 1, add 26.
Exam PatternsWhat examiners ask — read before attempting PYQs
Recognition
SSC CGL typically asks 1-2 questions on coding decoding
Common formats include
- If CAT = DOG, then BAT = ?
- In a code language, CHAIR is written as FKDLU. How is TABLE written?
- If MONDAY is coded as 123456, then SUNDAY is coded as?
ShortcutsUse these to save 30–60 seconds per question
#1 - Position Gap Method:
Find the gap between original and coded letters for first 2-3 letters. Apply same gap to remaining letters. This works for 80% of shift coding questions.
Worked ExampleSolve this step-by-step before moving on
1
Step 1
Find the pattern
H → K (H=8, K=11, difference = +3)
O → R (O=15, R=18, difference = +3)
R → U (R=18, U=21, difference = +3)
S → V (S=19, V=22, difference = +3)
E → H (E=5, H=8, difference = +3)
2
Step 2
Pattern identified = Each letter shifts +3 positions
3
Step 3
Apply to TIGER
T → W (T=20, +3=23=W)
I → L (I=9, +3=12=L)
G → J (G=7, +3=10=J)
E → H (E=5, +3=8=H)
R → U (R=18, +3=21=U)
Answer: TIGER = WLJHU
Worked Example 2:
Question: If in a certain code, BRAIN is written as CQBJO, how is SMILE written?
1
Step 1
Analyze each position
B → C (+1)
R → Q (-1)
A → B (+1)
I → J (+1)
N → O (+1)
Apply to SMILE
S → T (+1)
M → L (-1)
I → J (+1)
L → M (+1)
E → F (+1)
Answer: SMILE = TLJMF
Shortcut Trick #2 - Reverse Alphabet Method:
If A→Z, B→Y, C→X pattern appears, use formula: Coded Letter Position = 27 - Original Letter Position.
Shortcut Trick #3 - Number Coding Quick Check:
For number codes, first check if numbers represent alphabetical positions (A=1, B=2...). If not, look for arithmetic progression patterns.
#1 Most Common Trap/Mistake:
Students often assume the same shift applies to all letters without checking each position carefully. Many codes use different shifts for different positions (like Example 2 above). Always verify the pattern for at least 3 letters before applying to the answer.
Additional exam tip: If the pattern seems too complex, try working backwards from answer options - this often saves precious time in competitive exams.
Key Points to Remember
Coding Decoding involves finding patterns in letter/number substitutions
Most common pattern: Each letter shifts by same number of positions in alphabet
Formula: New Position = Original Position + Shift Value (adjust for alphabet limits)
Always check pattern in first 3 letters before applying to entire word
Reverse alphabet coding: A=Z, B=Y, use 27 minus original position
Number codes often represent alphabetical positions (A=1, B=2, C=3)
Mixed patterns have different shifts for different letter positions
Position Gap Method works for 80% of shift coding questions
If pattern is complex, work backwards from answer options to save time
SSC CGL typically includes 1-2 coding decoding questions per paper
Exam-Specific Tips
A=1, B=2, C=3... Z=26 is the standard alphabetical position formula
Forward shift formula: New Position = Original Position + Shift Value
Backward shift formula: New Position = Original Position - Shift Value
Reverse alphabet coding formula: Coded Position = 27 - Original Position
If coded position exceeds 26, subtract 26 to get correct alphabet position
If coded position is less than 1, add 26 to get correct alphabet position
Common shift values in SSC exams are +3, +2, +1, -1, -2, -3
Mixed position patterns typically alternate between +1 and -1 shifts
Practice MCQs
Coding Decoding — Practice Questions
15graded MCQs · easy to hard · full solution & trap analysis
In a code, consonants are replaced by the next consonant in the English alphabet, and vowels are replaced by the previous vowel in the English alphabet (A→U, E→A, I→E, O→I, U→O). What is the code for 'SMILE'?
Practice 2easy
In a certain code, each letter is replaced by the letter that comes 3 positions after it in the English alphabet. For example, A is coded as D, B is coded as E, and so on. If the word 'HELP' is coded using this rule, what will be the code?
Practice 3easy
In a certain code, the position of each letter in the word is swapped with the position of the letter that is equidistant from the end. For example, in the word 'LISTEN' (6 letters), L (position 1) swaps with N (position 6), I (position 2) swaps with E (position 5), and S (position 3) swaps with T (position 4). What is the code for 'FRIEND'?
Practice 4easy
In a code, each digit is replaced by (10 - digit). For example, 3 is coded as 7, and 5 is coded as 5. If the number 2468 is coded using this rule, what is the code?
Practice 5easy
In a code language, vowels (A, E, I, O, U) are replaced by the next vowel in sequence (A→E, E→I, I→O, O→U, U→A), and consonants remain unchanged. What is the code for the word 'HOUSE'?
Practice 6easy
In a certain code, each letter is replaced by the letter that comes 3 positions after it in the English alphabet. For example, A is coded as D, B is coded as E, and so on. If the word 'CAT' is coded using this rule, what is the code?
Practice 7medium
In a certain code, each letter is replaced by a symbol based on this rule: vowels (A, E, I, O, U) become ★, and consonants become ◆. Additionally, if a consonant is in the first half of the alphabet (B–M), it becomes ◆, and if it is in the second half (N–Z), it becomes ◇. What is the code for 'MODERN'?
Practice 8medium
In a code, each letter is assigned a number based on its position in the alphabet. Then, the number is multiplied by 2 if the letter is a consonant, or left unchanged if it is a vowel. For example, A(1)→1, B(2)→4. What is the code for 'TRAIN'?
Practice 9medium
In a certain code, each letter is replaced by the letter that comes 3 positions after it in the English alphabet. For example, A → D, B → E, and so on. Z wraps around to C. Using this rule, what is the code for the word 'LOGIC'?
Practice 10hard
A code assigns each letter a symbol based on this rule: vowels (A, E, I, O, U) are represented by their position in the vowel sequence (A=①, E=②, I=③, O=④, U=⑤), and consonants are represented by the symbol of the vowel that comes immediately after them alphabetically. For example, B comes after A, so B=①; D comes after E, so D=②. What is the code for 'KNOWLEDGE'?
Practice 11hard
In a coding system, each letter is replaced by a two-digit code: the first digit represents the position of the letter in the alphabet (A=1, B=2, ..., Z=26), and the second digit is obtained by reversing the first digit and adding 3 to it. For example, if a letter is at position 7, the first digit is 7, and the second digit is reverse(7) + 3 = 7 + 3 = 10, but since we need a single digit, we take 10 mod 10 = 0, so the code is 70. Using this rule, what is the code for the letter 'M'?
Practice 12hard
In a mirror-code system, letters are first converted to their reverse alphabet position (A↔Z, B↔Y, C↔X, etc.), then each resulting letter is shifted forward by 3 positions in the alphabet. What is the code for 'BRAIN'?
Note: Reverse alphabet means A=26, B=25, C=24, ..., Z=1 in reverse mapping.
Practice 13hard
In a coding system, each letter is replaced by the number representing its position in the alphabet, then each number is reversed (written backwards). Finally, all reversed numbers are added together. If the word is 'LOGIC', what is the final sum?
Example: A=1→reversed to 1, B=2→reversed to 2, ..., J=10→reversed to 01, K=11→reversed to 11, Z=26→reversed to 62
Practice 14hard
A code uses the following rule: consonants are replaced by the number of the next consonant in the alphabet, and vowels are replaced by the number of the previous vowel in the alphabet. Consonants are numbered 1–21 (B=1, C=2, ..., Z=21) and vowels are numbered 1–5 (A=1, E=2, I=3, O=4, U=5). What is the code for 'SYSTEM'?
Practice 15hard
In a hybrid code, each letter is assigned a value: A=1, B=2, ..., Z=26. The code for a word is created by: (1) summing the values of all letters, (2) reversing the digits of the sum, (3) subtracting 5 from the reversed sum. If the code for 'TRUST' is 89, what is the code for 'PRIDE'?
60-Second Revision — Coding Decoding
Remember: Check pattern in first 3 letters before solving completely
Formula: Shift coding = Original Position ± Shift Value
Trap: Don't assume same shift for all positions - verify each letter
Shortcut: Use Position Gap Method for quick pattern recognition
Technique: Work backwards from options if pattern seems complex
Quick check: A=1 to Z=26 for number-based coding questions
Time saver: Reverse alphabet uses 27 minus original position formula