At these curtains' protection military troops could easily move away, stick together, and attack. We usually understand smoke either like an autonomous object, perceptible despite the lack of precision as an entity, or like a medium. However, this quality implies the appearance of special relationships between objects. Limit supposes discontinuity, therefore differentiating between what lies on one side and on the other. On the other hand, discontinuities should not be total.
Limits are barriers as well as thresholds, more or less easily passed over.
The clich of the brave alpinist climbing on solitary peaks, beyond the clouds covering the valley of the daily life, ranges within this family of representations. We however chose the military image because on one hand it implies constructing the given limit, and on the other hand the groups separated in this way are not so strictly different, which facilitates the analogy with phenomena of the urban life. Obviously, the dialectics of opening and closing, of substance and form, of inner and outer, of public or private character of the space has generally not much to deal with this metaphor. We however can discuss the closing and the screening of those spaces, whose public character usually implies openness and accessibility. Long before the invasion of proper curtain-walls, the shop windows of the first chic restaurants, of the hotel halls and even of some agencies, banks or shops benefitted this treatment.