Plate V — Threshold

Structural intent

Define a boundary condition: a clear transition between zones (before/after) marked by a frame, gate, portal, or edge.

What must be observable

• A distinct boundary element (lintel, frame, doorway, arch, gate, bridge edge).
• Two separable regions (near/far, inside/outside, open/closed) with different properties.
• A readable crossing line where the transition occurs.
• Composition that centers the boundary as the primary subject.

Common failure modes

• Boundary implied but not visible (no framing geometry).
• Transition present but ambiguous (zones look identical).
• Multiple thresholds competing in the same frame.
• Threshold off-center, becoming incidental rather than defining.

UI / system translation (non-symbolic)

• Use for authentication, onboarding steps, and “mode changes.”
• In UI: step transitions, modal boundaries, section dividers with clear entry points.
• In system terms: explicit state transitions with guard conditions.
• QA: require visible boundary geometry and confirm two-zone separability.