Phone Keypad (Multi-tap) Encoding

Learn

Encode letters as (key, tap) pairs using the ITU E.161 phone keypad layout.

On a standard ITU E.161 telephone keypad, letters are grouped under digits 2–9: 2=ABC, 3=DEF, 4=GHI, 5=JKL, 6=MNO, 7=PQRS (four letters), 8=TUV, 9=WXYZ (four letters). In multi-tap encoding, each letter is represented by its key digit plus the number of presses needed to reach that letter on the key — A = 2-1, B = 2-2, C = 2-3, D = 3-1, ..., S = 7-4, Z = 9-4. The encoding is not case-sensitive and does not use the 1 or 0 keys (reserved for punctuation/space).

letter → (key, tap), where key ∈ {2..9} and tap is the 1-based index within that key's group

• HELLO → 4-2 3-2 5-3 5-3 6-3
• ABC → 2-1 2-2 2-3
• Z → 9-4 (the fourth letter on 9)

How to recognize it

• Digits 2–9 paired with small numbers 1–4
• No 0 or 1 digits appear as the first number of a pair
• The keypad layout 2=ABC ... 9=WXYZ is explicitly referenced

Common mistakes

• Forgetting that 7 and 9 each have four letters (not three)
• Starting the count at 0 instead of 1 for the tap number
• Assigning letters to 1 or 0 (those keys hold no letters)

Practice guidance

Coding & Decoding