A threshold is a point on the boundary between inside and outside that can be opened. A wide range of perceptions, movements and social encounters become possible there. As Norberg-Schulz notes, ‘the opening is the element that makes the place come alive, because the basis of any life is interaction’ (1971: 25). A threshold is also a restricted space; its design always constrains people’s behavior and their perceptions (Hillier and Hanson 1984). Many different architectural elements distinguish inside from outside and mediate people’s passage between them: doorways, turnstiles, colonnades, marquees, porches, terraces and stairways. Although these elements are designed to serve practical and ceremonial functions, thresholds present distinctive perceptual, behavioral, social and symbolic affordances which also give rise to a great variety of play.