In creating a work of art, knowing what to omit is no less important than knowing what to include. Sometimes, it's the voids, blank spaces, meaningful pauses or rests woven into artful compositions that acquire the greatest significance.

The same is true of cities. Their potentially continuous, uninterrupted fabric of streets, blocks and buildings is enriched by periodic perforation, distortion and subtraction. Omitting buildings or parts of buildings, or arranging buildings to surround or shape a space, introduces needed exceptions into urban street and block patterns.

Washington has many such spaces responsible for much of the city's visual character: the Mall, squares, plazas and circles. What makes these public spaces -- formed and functioning in so many different ways -- worthwhile omissions?

Squares and plazas offer relief, a chance to pause, slow down, stop and relax. Like great outdoor rooms lined by buildings and roadways, they allow people to gather and intense activity to occur. Urban wildlife -- countless pigeons, squirrels, an occasional seagull and dozens of other bird species -- and colorful vegetation remind us that the natural and man-made environments are inseparable and interdependent.

Some urban rooms, such as miniparks covered by grass, shrubs, trees and walkways, invite people to take leisurely strolls, eat picnic-style, sunbathe, voice protest, celebrate events, or just sit and talk. On a pleasant spring day, Farragut Square, bounded by I, K, and 17th streets NW, best illustrates this kind of space and activity, as does Lafayette Square north of the White House.

Other spaces, mostly paved with brick, stone or concrete, are punctuated only by landscaping. Here, if conditions are right, people also can meet, sit and talk, eat, ice skate in winter or watch the world go by. With its pool, steps, benches, food kiosk, umbrellas and outdoor furniture, Pershing Park at 15th Street and Pennsylvania Avenue NW attracts downtown workers and tourists whenever weather permits.

Some squares and plazas are primarily visual amenities, places to look at from an adjoining building window or a nearby street. They may be gardens, more decorative than functional, and rarely used for intense pedestrian activities. Often, they act as forecourts or visual settings for buildings along their edges or for monuments set within them. Hamilton Place, south of the Treasury Building at 15th and E streets NW, is one such space.

A few are sacred and ceremonial, spatial symbols of commemoration or sites for recurring ritual. Marked by elements such as fences, planting, statues and controlled paths that give clues to their sanctity, they have a compositional geometry that may be formal, balanced and carefully tailored to suggest that random intrusions would upset and spoil their stability and serenity. The spaces south of the White House and adjoining the Capitol's west front seem to inspire such feelings.

Squares and plazas may be part of a planned processional sequence enroute from one place to another, particularly from exterior to interior. Entrance plazas or courtyards can be created by setting back or carving out segments of buildings. From street and sidewalk, one proceeds into and across the courtyard or plaza, then enters the building.

Many federal edifices have been sited or shaped to achieve this sense of procession, which also heightens perceived building monumentality. Recall approaches to buildings near the Mall: the HUD building on 7th Street SW, the Old Executive Office Building, the Supreme Court or the Library of Congress.

Sometimes, however, building entrance plazas prove unnecessary, especially along commercial streets. Overscaled, underpopulated gaps in the street wall continuum serve only to reduce usable floor area. Maintaining the building-line facade wall and enriching the building's interaction with sidewalk through appropriate storefront and entrance designs might be preferable.

Urban open spaces facilitate views for those traveling through or near them, as well as for inhabitants of surrounding buildings. The facades of an entire block may be seen at once. Vistas along streets that radiate from or are adjacent to squares and plazas are improved dramatically because of increased viewing angles and viewing depths. You frequently can see the tops of other structures, both near and far, silhouetted against the sky beyond the immediately abutting buildings.

Well-conceived urban squares and plazas help orient us and tell us where we are in the city. As exceptions in the urban fabric, they stick in our memories more easily than dozens of streets, each of whose shape and character might be pleasant but undistinguished.

Any good open space in the city is really an amalgamation of recollectable images -- the form and proportions of the space itself, the character of surrounding buildings, the shape and texture of the ground plane, the views into and out of the space, and the activities associated with it. If a space is inadequately defined or contained by buildings and landscape, if its edges "leak" excessively, or if its form is unmeasurable and obscure, then its image may be compromised.

Ambiguity in purpose, plus inaccessibility, make some urban spaces seem residual, like leftover square footage belonging to no one. Such spaces may become nothing more than yard areas or squares to satisfy zoning requirements. If there is no reason to use urban spaces, they remain empty and lifeless. Or they might be transformed into parking lots, as within the Federal Triangle.

With enough activity at street level, almost any square or plaza, regardless of its shape, can be animated. But it also must be visible and accessible, an unavoidable waypoint for the urban traveler. Out-of-the-way, difficult-to-reach spaces have little chance of attracting streetscape activities or pedestrians. L'Enfant Plaza remains dead much of the time, even though it is a perfectly well-proportioned space, because few people go by it. And its locus of public activity is underground, leaving the plaza to vehicles.

Similarly, well-situated Western Plaza, bounded by 13th, 14th and E streets and Pennsylvania Avenue NW, is underutilized and easily forgotten because there's little reason to be there. Elevated and relatively featureless, it is hard to see, as well, except from the upper stories of nearby buildings. With Pershing Park next door, why go to this uninviting place?

Most good squares and plazas are crossroads, of which Washington's traffic circles are an extreme example. Circle interiors are landscaped islands totally surrounded by paving and automobile circulation. They are easier to see than to be in, because getting to the interior on foot can be life-threatening. Lively Dupont Circle -- ringed by trees, with the great marble fountain at its center -- is a notable exception because of its location at the crossing of so many major streets in a neighborhood rich in activity.

Like the squares of Franklin, Farragut, Mt. Vernon or McPherson, the circles -- Thomas, Sheridan, Westmoreland, Logan or Washington -- act as memorable spatial landmarks within the grid/avenue pattern. We tolerate their inefficiency as traffic facilitators because they and their encompassing buildings are unique place-makers.

Throughout history, and in most cultures, designers always have used courtyards, squares and plazas as primary elements in urban design. Who cannot help but love the wonderful piazzas and piazzettas of Rome, Florence or Venice; the squares of Paris and London, or the intimate, irregularly shaped spaces between blindingly white buildings on Grecian hillsides?

But these loved places, these significant voids, are typically filled with pedestrians who participate in daily and nightly theater of the streetscape. Without the need to shop, travel, socialize, work, recreate, dine or dwell in these urban spaces and their architectural entourage, they would become derelict and meaningless.

NEXT: City meets river