Every time I collide with a projecting toilet-paper holder, I am reminded of how critical inches can be. Whenever I fly on a full airplane, shoehorned into a seat seemingly designed only for slim-hipped, short-legged, people 5 feet 4 inches tall with no elbows, I recall how much inches matter.

Pay homage to the inch--and occasionally even the fraction of an inch. It can make a big difference in the comfort, efficiency and appearance of buildings, furniture and other items that we see, touch, occupy or use.

Think about heights of counter tops and wall cabinets, widths of corridors and toilet stalls or the skimpiness of head and leg room in automobiles. Consider the dimensions of openings, the arcs of door swings, the size of stair treads and risers, the length of lanes in parking garages and the width of parking spaces you try to squeeze into.

In homes and offices, just placing furniture can be a struggle if room dimensions, door placements and window locations are not quite right. Have you ever wondered how the frequently inconvenient location of light switches, thermostats, electrical and telephone outlets or heating registers in floors and walls was determined? How often have you wished that the position or size of something was only an inch or two different?

Given your personal dimensions, much of what's built may be slightly too close or out of reach, too tall or too short, too wide or too narrow. Many of the structures you routinely use, along with machines and devices produced by industry, frequently don't quite fit your nonstandard body.

Standardization is part of the problem. Industry builds or manufactures things using mass-production techniques and established industry norms but without knowing who the actual user will be. Whether a house or a hand tool, products typically are designed to fit someone who is ergonomically and demographically average. Too bad if you deviate from the average.

Most desks are 30 inches high, but you might be more comfortable with a desk that's higher--28 or 32 inches. Kitchen counters are 36 inches high, but a tall person wishing to bend over less might prefer 38 inches. By contrast, a short person might prefer working at a counter 33 inches high.

It is not economically feasible to custom-design everything to fit a potential user perfectly. And, generally, building in dimensional flexibility significantly adds to an item's complexity and costs. Making something fixed and rigid is much simpler than making it adjustable; adjustability requires special hardware, assembly details and perhaps electronics and motors.

A few industrial products, such as automobile seats and desk chairs, are adjustable, but theoretically many more could be. Imagine a lavatory vanity with a height that could be changed by several inches, possible only if flexible piping were reliable and accepted by building code officials.

Inches affect aesthetics as well as functional quality. The beauty of an object depends in part on compositional proportions--the relationships among the visible dimensions of the object.

The human eye and brain can discern, without actually taking measurements, certain features that depend on proportion: symmetry vs. asymmetry; horizontality vs. verticality; right angles; pure geometric figures, such as squares, circles and equilateral triangles; and visual rhythms created by the repetition of identical or similar elements.

Therefore, a dimensional misstep of an inch or two in an otherwise beautifully proportioned form can spoil the whole composition. For example, if an architect intends to center a set of French doors in a facade or on the central axis of an interior space but the contractor installs the doors a few inches off-center, the composition will look unbalanced and awkward.

In designing buildings, architects begin with rough sketches and models in which inches do not matter. Lines are fuzzy and dimensions are approximate. Nothing is irrevocably fixed, and the diagrams convey the essence but not the details of the idea.

Very soon thereafter, however, establishing realistic scale and dimensions is essential. Indeed, deciding the overall dimensions at an early stage ultimately determines--and limits--the final sizes of rooms, hallways, doors and closets. So the width and length of your bathroom may be because of design commitments made weeks or months before the bathroom was ever laid out.

Bruising your leg on a toilet-paper holder may be a symptom of inadequate care on the part of the person who positioned it. But it also may be the consequence of an architect's otherwise admirable concept yielding a floor plan with spaces just slightly too tight for their intended purposes.

In architecture, not "giving" or "budging" an inch takes on new meaning. When a design has been locked in, so that even the most minimal alteration reverberates adversely through the whole structure, the design is unlikely to change.

Keep this in mind the next time you wish for those few extra inches. They usually have to be stolen, at some cost, from somewhere and someone else.

Roger Lewis is a practicing architect and a professor of architecture at the University of Maryland.