Modus Fortis represents a Khunian shift in Sudoku strategy. Using just four basic tactics that give rise to just two molecular structures--Doubles and Pairs--that evolve into just two Families of complex structures (Exclusive Sets and Forbidden Sets) is all that is needed to solve every Sudoku. But just as E = mc2 is easy to visualize, there is a bit more to understanding the equation. The logical principle that drives the Solution Process is the Exclusive Disjunction: the Modus Fortis. This, combined with the Pairing Principle, guides the Sudoku-ka through the process of constructing a Solution from emerging Sets of Digits, identifiable as structural patterns. A concise notation system allows for recording every significant event in the Solution Process, just as in chess. Richly illustrated, this is a readable book for all levels of interest.