Fourth stratified-design tool from Wójtowicz's talk: the artsy-fartsy skill of choosing the right number of layers for your team at this point in time. Too few layers produces monoliths; too many layers produces low cohesion between them. Don't blindly accept frameworks that mandate '20 000 layers' (Trailblazer as the caricature) or advocates who say you need none (DHH as the caricature). This intuition cannot be taught but is learned by repeatedly drawing call graphs and iterating on designs.