Strategic fit
Strategic fit is the mutual reinforcement among a firm's activities, so that the whole system is harder to imitate than any single part.
A good strategy is not a list of good things to do. It is a set of choices that reinforce one another.
Strategic fit is the degree to which a firm's activities support and strengthen each other, so that the whole system delivers more than the sum of its parts and is far harder to imitate. Porter argued that fit, not any single activity, is the deepest source of sustainable advantage.
Why fit beats best practice
Individual best practices spread quickly. A rival can copy a clever pricing tactic, a slick logistics method, or a generous service policy in isolation. What it cannot easily copy is a system in which dozens of activities are tuned to one another. Each piece on its own looks ordinary; the advantage lives in the interlocking.
This is why benchmarking can be a trap. Adopting the best version of each activity from different leaders produces an incoherent collection that fits nothing. Fit requires that activities be chosen to complement the firm's particular position, not to be individually optimal.
Orders of fit
Porter distinguished simple consistency between each activity and the strategy, reinforcement where activities amplify one another, and optimisation of effort where coordination removes waste and redundancy. The higher orders are where imitation becomes hardest, because a competitor must replicate not one choice but the relationships among many.
The discipline of saying no
Fit imposes a cost most firms resist: it requires turning down opportunities that do not match. A position built on fast, low-cost, no-frills service is undermined the moment the firm bolts on premium options to chase another segment. Trade-offs are what make fit durable, because they force straddling rivals to choose between imitating the system wholesale and not at all.
The practical test of fit is whether the activities tell a single, consistent story. If you can add or remove a major activity without anything else needing to change, the system is loosely coupled and easy to copy. Tight fit is uncomfortable and constraining, which is exactly why it lasts.