CORNICE MOULDINGS serve a function that few people, perhaps, wish to be served nowadays; namely, the precise articulation of wall and ceiling.

One grand theory of the present century is that the less anything is defined, the better. This permits, at all levels of civilization, an interchange of parts. Thus Cabinet members, corporation chairmen and consultants of all types are utterly interchangeable and so (in the field of design) are kitchens, bedrooms, libraries and the like.

The ideal, toward which we annually soar, is to eliminate enclosed space altogether and perhaps heat up the forest a bit so that everyone is completely free.

There would be, if we all lived outdoors like porcupines under a hedge, no tyranny of little boxes or other sharply defined spaces, and mankind (as this theory goes) would at last be limitless in potential, as the new horizons gazed straight back at us without barriers of any kind.

At the moment, however, when many people still live in boxes, ceilings are generally white and walls are commonly colored. Where ceiling meets wall there is obviously a sharp contrast which many people like.

If you get your mind and your eyes used to it, there is no reason a crimson wall cannot meet a white ceiling quite frankly, without embarrassment or ceremony.

But to some people, the effect is easier to live with, and more stisfying and neat, if a cornice is used to rest partly on the wall and partly on the ceiling. A cornice says (silently, unless it is poorly installed) "this is the wall and this is the ceiling and they are separate."

It is a fashion to ignore the difference, and in many houses the ceiling is painted the same as the wall.

I cannot myself think of anything more dismal than a room painted solid rich blue on walls and ceiling alike, but if the effect is one that people like, they should certainly have it.

Yet even today there is a type of human that likes to emphasize the line where the vertical walls stop and the horizontal ceiling begins. To them it looks "right."

If they find themselves in a house or apartment without cornices or crown mouldings (a cornice consists of a crown moulding and usually a good many other mouldings, and is merely a more elaborate treatment of the simple crown, which diagonally bridges wall and ceiling) they feel a needed weight is missing.

They will, many of them, lose no time looking about for cornices, and will soon learn they have three choices:

There is carved or (more realistically) machine-milled wood.

There is plaster, either designed to order in custom moulds or in standard designs.

There are composition materials, such as urethane, a by-product of the petroleum industry.

Needless to say, a cornice can be made of alabaster, limestone, bronze, iron or virtually anything else ever used as a building material, but in general the materials mentioned are the ones to be considered.

Wood cornices can be extremely simple, consisting of only one piece, which is curved in profile and rests partly on the wall, partly on the ceiling. It can cost less than a dollar a foot, and is usually installed by a careful (one hopes) craftsman who knows when to use nails and when to use more complicated devices sutiable for masonry walls (where ordinary nails will not do).

Usually there is more than one piece. Each is different in profile, and the various pieces are simply nailed one after the other, to produce an ultimate profile of great complexity and richness.

Such lumber yards as Gaithersburg (Md.) Lumber and W. T. Galliher & Brother, Inc., are among the many who can supply ready-made cornice members or have them made to order.

It should be clear that the more pieces there are in a cornice profile the more expensive it will be, and also (a thing sometimes overlooked) the more time it will take the carpenter to install it. A wooden cornice of several members that costs prehaps $1.20 a foot may take a good many hours of a craftsman's time to install, and often the lumber company can refer the customer to men who can do the installation work, and give an idea how long the job may take.

The most beautiful cornices of the 18th century in Virginia, Maryland and Carolina were of wood. They were not cheap when they were made, and although they can readily be copied in wood today, the price would seen prohibitive or downright wicked to most people.

One thing the client for wooden cornices should do is see the cornice before going ahead with it. If it is decided that a line of dentils (those little blocks of wood occurring at regular close intervals somewhat like teeth) would be nice, it is important to actually see them. Some dentil members are made of wood no thicker than cardboard and the result, to my eye, is quite absurd since the shadow that is cast by the dentils is so light as to be all but invisible. On the other hand, the client may want just a hint o a dentil, so to speak, and will find the one-eighth inch depth just the thing. The only trouble comes when one is thinking of dentils an inch deep and they come out an eighth of that.

Another thing about wooden cornices from lumber yards, it must be remembered that most lumber companies derive an extremely small percentage of their income from cornices. They are not likely to break their necks going over designs - especially if they think they are doing you a great favor to begin with.

An alternative to wood is plaster. Cornices may be as simple as a three-inch deep band of acanthus leaves with neat mouldings above and below, or can be more exuberant and lavish in design than anything made of wood.

A firm like Hytla & Hart, 3019 K St. NW, for example, can reproduce the elaborate cornices of Monticello, Jefferson's house at Charlottesville, or turn out (and install) a simpler profile suitable for rooms with only eight-foot ceilings. (The small acanthus cornice can be made and installed in ordinary living rooms for about $3 a foot, I was told.)

A room full of chimney breasts sticking out and bay windows and curved walls and other irregularities of outline will require more installation time, and therefore cost more, than a room of rectangular or square shape.

One beauty of plaster is that it can be cast for curved walls easier than wood can be bent and joined.

Small cornices for ordinary Washington living rooms weigh about two pounds a foot. A special-formula plaster is used, so weight is not generally any problem at all.

In plaster, as with any other material, it is important for the client to make clear (usually the plaster contractor will come out and see the room for himself) just what he wants and where he wants it, and to be sure whether the quoted price includes the installation or not.

Finally there are such plastic cornices as those made of urethane, and like plaster these cornices make possible designs impossible in wood.

A four-member counice (a cyma or two, a dentil course, and so on) would require four pieces of wood, but is cast in one piece in urethane.

One company that makes these, Focal Point, Inc., 3760 Lower Roswell Road, Marietta, Ga. 30067, distributes a printed folder and, on request, samples.

I suspected this material was rather like a plastic bucket, but discovered it is the density of white pine and looks like a plastic wood. It comes from the factory primed white or deep beige, ready for finish coat.

Like wood, the corners (where one wall meets another) are to be mitred. It weighs only a few ounces per running foot, and is attached with a special mastic and occasional finishing nails. Theoretically the purchaser can install it himself, or hire a carpenter to.

There are about 10 designs available, of varying degrees of ornateness, some of them simply duplicating wooden mouldings but others with grape vines an d shells. Two merits are the light weight and relative simplicity of putting it on the walls, since in even a four-member cornice the whole thing is cast in one piece and handled like one board.

Such mouldings run about $2.50 a foot, not counting installation.

Perhaps we have said too much about cornices for ordinary-sized houses. If anyone happens to have 20-foot ceilings and an incurably baroque or rococo soul with a sense of humor and a bit of money, there is no reason whatever for holding back. Plaster is an ideal medium for cherubs, goddesses, swags of fruit and Chinese curvatures. Properly disposed with brave abandon from the chandelier down to the chair rail, the effect will be such that nobody will say "this room is a bit severe and plain."

As for neo-classic mouldings, such as Jefferson was fond of, they are undoubtedly awe-inspiring, since Jefferson was all his life terrified of making anything too delicate in scale. He had a horror of skimpiness. I remember one night in Charlottesville, sleeping in a bedroom he designed with his customary massive cornices, and wondering how many years plaster held up without falling. When Jefferson's cornices do at last full, thousands with die. Still if you like them . . .