SUDOKU HARD - HARD SUDOKU FUNDAMENTALS EXPLAINED

Quads tend to be rarer, plus they’re less handy in a method because they don’t Allow you to slender down a large number of candidates all at once, however, if you spot 1, it’s well worth investigating. Solving single squares might also turn quads into triples or pairs, so don’t ignore these!​​​​​​​​​​​​​​​​â€