Open plan living is a vast improvement on life in a box, or in a series of boxes.
 
But open-plan living still requires a delineation of spaces within the larger space -- it is just more difficult for the designer to ensire the right degree of separation, and the right amount of integration with the main space.  In a well-designed open space, each space will have its own character, and will flow easily and comfortably into other spaces. It should feel natural, not forced, with each space retaining its own character with the laregr space, but still being a part of it.  Done fproperly, the 'separation' of spaces gives the right feelings of liberation or enclosure appropriate to the space.
 
It is important to define each space within the larger whole in a subtle manner so as to avoid there simply being a space like 'one large barn.'  Some popular methods of achieving this separation are:
 
* different floor coverings in different space;
* changes in floor level between spaces;
* furnishing layouts;
* the use of sliding or folding 'screens';
* the use of room proportions and wall returns to break up a larger space into smaller sub-spaces;
* posts and the like between spaces;
* changes in window and door layouts, eg., a french door layout in one part of a space, and perhaps clerestorey windows in the other;
* changes in ceiling height, including the use of lowered ceilings and lowered ceiling decks.
 
The method by which the separation is achieved is usually dependent on the extent and quality of the spacial separation required. 
 
Ceiling decks offer a particularly subtle way both to 'frame a space' and to divide one space from another, but also -- by complex cognitive means -- lowered ceiling decks that 'framed' a space help to make that space and those around it appear larger than they are.  A lowered ceiling deck is simply a lowed or independent part of a ceiling, usually horizontal in the manner of an above-ground 'deck.'
 
Below are some examples of lowered ceiling decks and ceiling dividers using different styles, and from different eras. A ceiling deck is limited only by your imagination: