Four Kinds of Constraints
Constraints are powerful clues that limit the set of possible actions, letting people determine the right action even in novel situations. Illustrated by the Lego motorcycle (15 pieces assembled correctly with no instructions). Four universal classes:
Physical — limits from the physical world (a large peg won't fit a small hole; a windshield fits only one place). No training needed. Most effective when visible/interpretable before acting (otherwise they only block a wrong action after it's attempted). Example failure: cylindrical batteries lack physical constraint on orientation.
Cultural — each culture's set of allowable actions for a situation (how to behave in a restaurant; red = brake light, in the rear). Represented mentally as schemas / scripts (Schank & Abelson) / frames (Goffman). Cultural constraints change over time (yellow once = European headlight).
Semantic — constraints from the meaning of the situation (the rider must face forward; the windshield protects the face → in front). Rely on knowledge of the world; can change as technology changes meaning.
Logical — relationships of logic (the one leftover Lego piece goes in the one remaining place; a leftover faucet part means you missed a step). Natural mappings work by providing logical constraints (left switch → left light).
Conventions
Conventions are a special kind of cultural constraint on behavior — sometimes codified into standards or laws (which side of the road; which fork to use). Violating conventions marks you as an outsider and can disrupt interaction; changing them is hard (see "Case: Destination-Control Elevators (when conventions change)").
Relationship
Constraints are one of the The Six Fundamental Principles of Interaction; the strong form is a forcing function. They are also knowledge in the world (physical/logical) and in the head (cultural).
Confidence
1.0 — explicitly enumerated.
Sources: ["DOET ch.4 'Four Kinds of Constraints' p.125-131 (Lego motorcycle)"]