You know the way you form an impression of someone just from the way they look? Your eyes take everything in almost instantly - the face, the clothing, the posture - and your brain forms an opinion. As with people, first impressions of a house are instinctive, immediate and powerful. Maybe you've driven by a certain house, done a double-take, backed up, and said to yourself, "Now, I like that house." Or, perhaps you've browsed through a book or magazine - maybe even this one - when, flipping through, you suddenly stopped, turned back a few pages, and said to yourself, "Now, I like that house."

From the outside, a house is simple, really: Walls, and a roof, with pieces added or taken away. So, what is it that makes you go back, to the house on the street or the picture in the magazine? Why does one house appeal to you more than another? Architects call it design. Real estate brokers call it curb appeal. You like it? What is it?

If you ask most people to draw a house, they will probably draw a five-sided figure - the ground, two side walls, and the two surfaces of a pointed roof. The roof shape is a gable, and it's almost a universal symbol for shelter. The majority of American houses sit under pitched roofs of one kind or another. If you clip the end of a gabled roof off at an angle you have a hip roof. Take the same gable and bend each side down its length and you get a roof traditionally associated with barns, the gambrel. The simplest of all pitched roofs is a shed, a flat roof surface higher at one side.

Finally, there's the flat roof itself, which doesn't seem to have any other name. Despite distinguished examples of flat-roofed houses in the 1920's International Style, 1930's Art Moderne, or the Pueblo Style of the American Southwest, most people seem to prefer the more traditional shapes. This isn't to say a flat roof is required for "modern" homes. The Prairie Style of Frank Lloyd Wright and his Chicago contemporaries, and the Western Stick, or Bungalow, Style practiced by the Greene Brothers in California - all in this century - make use of gabled and hipped roofs, as do the non-traditional houses of many architects practicing today.

The shape of a roof determines to a great extent the style of a house and how you react to it. It can trigger associations in your mind - with a fairy tale castle, a childhood home or a fantasy dreamhouse. The depth of a roof overhang can affect the sun and shade inside. Usable upper story and attic floor space is in part a product of the pitch and height of the roof.

Rooftop ornament is important, as well. If you see what looks like a balustrade up at the peak of a roof, it may be bordering a widows walk. This rooftop platform is said to have been used by anxious New England women scanning the distant horizon for the tall ships of their long absent seafaring husbands. While the original use may be obsolete, such a platform is still pleasant for taking in the harbor view, the nighttime constellations or city lights. You can also enjoy the view from a rooftop belvedere, which in fact means "pretty view" in Italian. You might call it a rooftop gazebo. (What's that? A summerhouse, or backyard garden structure like the kind from which brass bands once serenaded the neighbors gathered on the village green.)

The roof can be a gateway for light and air. On many older homes and barns you will find a cupola, a miniature house with louvered sides for attic ventilation, or windows for light. There's one atop George Washington's home, Mount Vernon (and on most Howard Johnson's restaurants). Function aside, a cupola, or for that matter other kinds of rooftop ornaments, like weathervanes and pinnacles, can "finish" a roof visually, like flowers in a vase or a hat on a head.

A dormer, (George Washington had some of these too) can perform much the same job as the cupola. In addition to ventilation and light, though, a dormer can make usable space of the low-roofed edge of the attic. Dormers come in all shapes and sizes, self descriptively name things like eyelid , hipped,, and shed.

A monitor is almost a dormer, but not quite. While it won't recover floorspace, it can get you daylight you might have thought unavailable. Imagine an illuminating periscope, popping up through the roof to get light back down to the interior of the house, a kind of directional skylight. If you have a breakfast room on the west side of your house and you want morning sun, put an east-facing monitor overhead.

The skylight itself is the simplest form of roof opening, actually just a window in the roof. Alone it gives little control of the sunlight, and can create overheating problems. But these can be handled by tinting, exterior shading, careful placement or some kind of interior shielding. A transparent or translucent roof opening can be quite dramatic, and versatile. Even an entire roof of glass is possible. There is even a skylight product, on the market now, which not only opens for ventilation but actually converts to a miniature balcony.

Like roof openings and accessories, conventional windows - the kind you find in walls - come in all shapes, sizes and types. Ubiquitous and endlessly various, these holes in the wall contribute mightily to our impression of a house. Size, shape, location and detail configuration all play a part.

The size of glass window panes, called lights, or glazing, is often taken as the sign of a home's age. Window openings divided into grids of wood and glass - divided lights- do indicate that a house is older, or at least trying to look "traditional". In pre-industrial economies the technology of making a large expanse of glass was, when available, costly. So, while there are 19th Century Chateau and Queen Anne Victorian homes with large windowpanes, they are the exception.

Homebuildiers today sometimes prefer divided lights. Besides the venerable appearance, the mullion grid can create interesting patterns of sunlight and shadow. It can give a comforting sense of protection from the outside without cutting off the view. While "snap-in" grids are available to provide old world feeling with "snap out" easy-to-clean convenience, they don't have the hand-made quality which provides the charm of the real thing.

Window locations are critical to the way a house looks and works. A window located high in a wall, called a clerestory. is especially useful if you need light or view and privacy, or if you need wall space for artwork or furniture. Built-in furniture can be combined with windows in a variety of ways, as with the window-seat incorporated in the three sided projection from the side of the house, the bay window. If the sides of the bay are angled, it becomes a cant window. If the projection is rounded, the name is bow or compass window. If the projection is corbelled (sticks out supported by angled walls) beyond the perimeter of the foundation, the window is an oriel. All take advantage of sidelong view, be it a snatch of the San Francisco Bay down the urban street, or the Rocky Mountain vista outside the sitting room of a cottage on a Montana country lane.

A few years ago a Japanese house appeared in a magazine with windows placed to form a human face. The effect wasn't visible on the inside of course, although light and air were still efficiently delivered. The late architect Charles Moore humorously described a row of openings he designed in one of his own homes as "falling windows" because each was placed at a greater angle than the previous one, as if tumbling, outside and in. Windows grouped together in horizontal bands provide ribbons of light inside, and on the outside can make the roof appear to float. Oversized windows placed symmetrically about a front door as in a Southern plantation house can be imposing, even intimidating, while serving the parlor and the dining room.

Even among ordinary rectangular windows, size and proportion affect our perception. A very narrow window can be unfriendly, even hostile, like the defensive openings for archers in medieval castle walls. Of course shapes can vary. A Palladian window, named for the 16th century Italian Renaissance architect Andrea Palladio, is frequently arched in the center of its three sections. Arched windows themselves can take different forms, like Gothic (pointed) , or roman (semi-circular). There is even the flat arch, which isn't always flat exactly, just almost flat. Rounded arches tend to soften the appearance of a facade, recalling a rustic Spanish rancho, while the pointed ones conjure medieval castles or cathedrals, less calming images for some.

Even window operation has an effect on the message conveyed by a house. Double hung windows go back as far as houses do on this continent, and can help give an historical look. They slide vertically past each other in a channel incorporated in the side, or jamb. You may have lived in a house with an inoperable double hung window, painted closed or with a jammed counterweight. In newer models, the sash (the wood frame that holds the glass in all windows) is pressure fit into the jamb - no counterweights to get in the way, though the painters still can.

Casement windows are ancient among window types. They swing open like doors. They can swing in, or out, though if you want screens you are limited to the inswinging kind - the screen would be in the way getting your hand on the window to push it out. You can solve the screen problem by using widely marketed crank operated casements, though the intermediary mechanism takes away some of the old world look and feel.

Casements can come in pairs, or singly. Other windows of one-piece appearance include awnings, and hoppers, also called hopper vents or hopper ventilators. Also, there is the fixed window. (Other names are, fixed light, deadlight, fast sheet, stand sheet or fixed sash. You may know it as a picture window, a piece of plate glass set directly into a fixed frame with no moving parts).

Sliding, and jalousie, also called louvered, windows, have a bad reputation in some circles partly because they are often used in inexpensive construction. Because they came into wide usage in modern times, they have no historical connotations. But used properly they can look good and work well. Sliding windows are an inexpensive solution to tight spaces where there is no room for a casement's swing or crank. Jalousies, because they open over the entire area of installation, provide maximum ventilation when opened (though poor insulation and security when closed).

With all these architectural elements to choose from, the question arises: When it comes to your own house, what are you supposed to use, and when? Well, there are no hard and fast rules. While the answer, "If it looks good, do it" is a little too glib, it contains some truth. It also implies another question: "What looks good?" And around you go.

It depends on your criteria. If it's accuracy you're looking for in a particular architectural style, you'll need to consult the history books. It is possible though, to have an attractive house that combines elements from different styles and eras. The trick is to do it tastefully. The key here may be in another time-worn adage: "Keep it simple." If you have three dormers, maybe they should all be of one type, as in shed, or gable. If you're planning on divided light windows, perhaps they should all be divided light. It might be a good idea to to limit the overall number of rooftop projections.

Maybe...? Perhaps...? Might...? Definitely! In house design, it really is true that rules are made to be broken. In a rooftop landscape of gabled dormers, a shed roofed monitor could provide welcome visual relief. In a house of divided light windows, the one, giant picture window would be an exciting focal point. And a ridge topping cupola might be the perfect cap for a roof full of flat skylights.

So, why does one house appeal to you more than another? Call it design, call it curb appeal, call it a shed roof with an eyebrow dormer and double hung windows. You'll probably know it when you see it. And you'll know what it is.

In conversations with architects around the country about the things clients like and request, some patterns emerged. Marty Schwartz, who has taught at the Universities of Oregon, and Plymouth, England, and practices now in Ann Arbor Michigan, describes the desire to "recall a certain sense of life."