T1 - Direct Arithmetic: Solve equation with 2 knowns → 1 unknown (e.g., ? + 5 = 12)
T2 - Single Candidate: Cell has only one valid value across all constraints
T3 - Cross-Equation: Solve by analyzing multiple intersecting equations together
T4 - Linear Systems: Solve 2x2 or 3x3 system of equations simultaneously
T5 - Dependency Chains: Back-substitute across chains of 3+ linked equations
T6 - Logical Guessing: Try value, propagate, backtrack if contradiction found
Difficulty is rated by which techniques are needed, not just how many cells are empty