Textbook of Usability

Introduction

The preceding chapters drew their laws primarily from experimental psychology and information theory — controlled laboratory studies of human performance. But there is a much older tradition of design knowledge, accumulated over millennia in the practice of architecture. The architects of classical antiquity, the Renaissance, and the Enlightenment developed principles of proportion, symmetry, and organisation that were not discovered through controlled experiments but through centuries of practice, refinement, and what amounts to a cultural process of natural selection: buildings that worked were imitated; buildings that failed were abandoned. This chapter argues that these architectural principles constitute a legitimate and valuable source of evidence for design — different from experimental evidence, but complementary to it. Where Fitts's Law tells us how fast a hand can reach a target, Vitruvius tells us how spaces should be proportioned for human comfort. Both are statements about the relationship between designed objects and human beings.

Vitruvius and the Classical Foundation

Marcus Vitruvius Pollio, writing in the first century BCE Pollio, -30, articulated three qualities that every well-designed structure must possess:

• Firmitas (firmness/durability): the structure must stand up and last
• Utilitas (utility/function): the structure must serve its intended purpose
• Venustas (beauty/delight): the structure must be aesthetically pleasing

Symmetry and Eurythmia

Vitruvius distinguished between symmetry (the mathematical relationships between parts) and eurythmia (the visual impression of those relationships when perceived by a human observer). A building might be mathematically symmetrical but not appear so when viewed from certain angles — in which case Vitruvius recommended deliberate adjustments to the proportions (what the Greeks called optical corrections) to ensure that the perceived result matched the intended design.

The Golden Ratio and Proportion Systems

The golden ratio (φ ≈ 1.618) appears throughout classical architecture and has been claimed to underlie human aesthetic preference. The evidence for a universal preference for the golden ratio is mixed — some studies find a mild preference for rectangles with golden proportions, while others find no clear preference or preferences that vary with culture and context. What is less controversial is that systematic proportion — using a consistent ratio to derive measurements — produces visual coherence. Whether the ratio is the golden ratio, the square root of 2, or a simple modular grid, the use of a consistent system prevents the arbitrary mishmash of dimensions that makes a design feel uncoordinated.

The Classical Orders

The Greeks developed five orders of architecture — Doric, Ionic, Corinthian, Tuscan, and Composite — each defined by a complete system of proportions. The column height, diameter, spacing, entablature height, and ornament are all derived from a single module (typically the column's base diameter). This modular approach ensures that every element is proportionally related to every other. The orders are, in essence, design systems: complete specifications that produce internally consistent results. Modern design systems in software — Material Design, Apple's Human Interface Guidelines, IBM's Carbon — serve the same function: they define a module (an 8-pixel grid, a type scale, a colour palette) and derive all measurements from it.

Palladio and the Four Books

Andrea Palladio (1508–1580) codified his design principles in I Quattro Libri dell'Architettura (The Four Books of Architecture) Palladio, 1570, one of the most influential architecture treatises ever written. Palladio's buildings are admired for their clarity, symmetry, and proportion.

Room Proportions

Palladio specified seven ideal room shapes, defined by the ratio of length to width:

1. Circular
2. Square (1:1)
3. √2 rectangle (1:1.414)
4. 4:3 rectangle
5. 3:2 rectangle
6. 5:3 rectangle
7. 2:1 rectangle These ratios are not arbitrary; they correspond to musical harmonic intervals (octave, fifth, fourth, major third), reflecting the Renaissance belief in a deep connection between visual and auditory harmony.

Symmetry and Hierarchy

Palladio's buildings are organised around a central axis, with rooms arranged symmetrically on either side. The largest, most important room occupies the centre; subsidiary rooms are smaller and arranged hierarchically. The entrance provides a clear visual axis that draws the visitor inward. In interface design, the analogy is direct: a well-structured page has a clear visual hierarchy (the most important content is largest and most central), a logical organisation (related elements are grouped), and a clear entry point (the user's eye is drawn to the right starting point).

The Portico and Threshold

Palladio's buildings typically feature a portico — a formal entrance that marks the transition from exterior to interior. The portico serves both a functional purpose (shelter) and a psychological one (signalling arrival, creating a sense of entry). In software, the equivalent is the landing page, the login screen, or the onboarding flow. These thresholds orient the user, set expectations, and create a sense of entering a defined space. A well-designed threshold reduces disorientation; a poorly designed one (or its absence) drops the user into the middle of an interface without context.

James Gibbs and Rules for Drawing

James Gibbs (1682–1754), the Scottish architect responsible for St Martin-in-the-Fields in London and the Radcliffe Camera in Oxford, published Rules for Drawing the Several Parts of Architecture Gibbs, 1732. Unlike Palladio's theoretical treatise, Gibbs's book was a practical manual: a set of rules that a builder of moderate skill could follow to produce classically proportioned buildings. Gibbs's contribution was the democratisation of design knowledge — converting the implicit expertise of master architects into explicit rules that could be followed by anyone. This is precisely what design systems, pattern libraries, and heuristic checklists do for software design: they codify expert knowledge into forms that non-experts can apply.

Christopher Alexander and the Pattern Language

Of all the architects in this chapter, Christopher Alexander (1936–2022) is the one whose work has had the most direct influence on software design. In Notes on the Synthesis of Form Alexander, 1964, Alexander set out a theoretical account of design as the resolution of competing requirements — a search for a form that simultaneously satisfies all the forces acting on it. In A Pattern Language Alexander, 1977, co-authored with Sara Ishikawa, Murray Silverstein, and colleagues at the Center for Environmental Structure, he translated that theory into practical guidance: a collection of 253 interlocking patterns, each describing a recurring problem in building and town design together with the core of a solution that could be reused in countless variations. Alexander's patterns range across scales, from the layout of regions ("The Distribution of Towns," "Mosaic of Subcultures") through the arrangement of buildings ("Building Complex," "Wings of Light") to fine-grained details ("Window Place," "Alcoves," "Stair Seats"). Each pattern names a recurring situation, describes the empirical observation or experience that motivates it, and offers a generative rule rather than a fixed prescription. The patterns are explicitly connected to one another: larger patterns are elaborated by smaller ones, and the collection forms a loose grammar from which a designer can assemble a coherent whole. This idea — that design knowledge can be captured as a network of interrelated, named, reusable patterns — was taken up almost directly by software engineering and HCI. The "Gang of Four" book Design Patterns Gamma, 1994 borrowed Alexander's structure and terminology for object-oriented software; Ward Cunningham and Kent Beck's work on programming patterns and the first wiki drew explicitly on A Pattern Language; and the HCI community has produced numerous pattern libraries (Tidwell's Designing Interfaces Tidwell, 2020, Van Welie's pattern collection, Yahoo!'s Design Pattern Library, and the living pattern libraries maintained by modern design systems such as Material and Carbon). The idea that a usable interface is an assembly of well-understood, named components each resolving a recurring tension between conflicting requirements is Alexander's contribution, borrowed and adapted.

Architectural Principles Applied to Software

Hierarchy and Spatial Organisation

Architecture organises space hierarchically: public rooms are larger and more prominent; private rooms are smaller and more secluded. Circulation paths (corridors, stairs) connect them. In software, the information architecture serves the same function: navigation structures, page hierarchies, and content organisation create a spatial structure that users navigate.

Wayfinding

Kevin Lynch's The Image of the City Lynch, 1984 identified five elements that people use to navigate urban environments: paths, edges, districts, nodes, and landmarks. These same elements appear in software navigation: breadcrumbs (paths), borders and dividers (edges), grouped sections (districts), hub pages (nodes), and distinctive visual features (landmarks).

Rhythm and Repetition

Classical architecture achieves visual rhythm through the repetition of elements — columns, arches, windows — at regular intervals. This rhythm creates visual coherence and makes large structures comprehensible. In interface design, consistent spacing, repeated component patterns, and regular grid structures serve the same function.

The Relationship Between Ornament and Structure

The modernist architect Adolf Loos declared "ornament is crime" (1908); the postmodernists restored decorative elements. The debate mirrors a tension in interface design between minimalism (remove everything non-essential) and expressive design (visual richness enhances engagement and delight). The classical position — Vitruvius's — is that ornament should be structurally honest: it should reveal or emphasise the underlying structure, not disguise it. A column capital is ornamental, but it marks the structural joint between column and entablature. Applied to interface design: visual embellishments are appropriate when they reinforce the structure and hierarchy of the information; they are harmful when they obscure it.

Evolved Practice as Evidence

The architectural principles discussed in this chapter were not discovered through controlled experiments. They evolved through a process analogous to natural selection: buildings were designed, lived in, evaluated by their inhabitants and successors, and either imitated or abandoned. Over centuries, this process filtered out designs that failed to serve human needs and preserved designs that succeeded. This evolved practice constitutes a form of evidence — not experimental evidence with controlled variables and statistical significance, but observational evidence accumulated over a very large number of trials (buildings) and evaluated by a very large number of judges (inhabitants) over a very long period (millennia).