When three digits can only appear in the same three cells within a unit, all other candidates can be removed from those cells.

A Hidden Triple occurs when three specific digits within a row, column, or block can only appear as candidates in the same three cells. Since those three digits must occupy those three cells, all other candidates can be eliminated from them. This is the three-digit extension of Hidden Pairs and is required for Expert difficulty and above.

Principle and Conditions

List the candidate positions for all digits in a unit. If three digits all exist only within the same subset of three cells, a Hidden Triple is formed. Those three cells must contain those three digits, so all other candidates are eliminated. For example, if digits 2, 5, and 9 can only appear in R4C1, R4C4, and R4C7 within a row, eliminate all candidates except 2, 5, 9 from those cells.

Discovery Difficulty

Hidden Triples are extremely difficult to spot because the three cells typically contain many other candidates that obscure the triple relationship. Focus on digits with candidates limited to 2-3 cells, then check whether any combination of three such digits shares the same three cells. Use this as a last resort after exhausting simpler techniques.