The entries here are what designers and engineers tell each other: working rules and a handful of sayings. The rules share a humble starting point, that people will use a system as they are rather than as its designer hoped. The sayings share a warning, that having all the parts of a system is not the same as having a system.
Design for people as they are
The most practical rules start from the user. The Principle of Least Astonishment asks that a component behave as its users already expect, not as its author finds elegant. Jakob's Law explains why: users spend most of their time on other products, so they bring those products' habits with them. Poka-yoke, from Toyota's production system, goes further and designs the wrong action out altogether, as with a plug that fits only one way.
Where complexity goes
A second set of rules concerns complexity. Tesler's Law holds that every task has some complexity that cannot be removed, only moved, so the question is whether the designer or the user carries it. The Zero-One-Infinity Rule warns against arbitrary limits, which always turn out too small. Toyota's Muda, Mura, Muri distinguishes three kinds of waste, and observes that removing one, such as idle time, can create another, such as overburden.
Judging and changing systems
Two rules are about how to judge and reform systems, and they point in different directions. Stafford Beer's POSIWID says the purpose of a system is what it does, so a system that consistently produces a result should be judged by that result, whatever its mission statement says. Popper's Piecemeal Social Engineering says reform should proceed in small steps that can be tested and reversed, because large designs cannot be. Read together, they suggest looking hard at what a system actually does and then changing it gradually.
The sayings
The figures carry the group's scepticism. All the Right Notes, from the Morecambe and Wise sketch with André Previn, names the failure of having every component in the wrong order. Deus ex Machina names the rescue from outside that a situation could not have produced for itself. The social version of Le Chatelier's Principle observes that systems push back against disturbance. Akin's Laws of Spacecraft Design collect an engineer's hard lessons, and Jugaad, the Hindi word for improvised sufficiency, records what happens when there are no parts to spare at all.
Where it applies
The user-facing rules apply well beyond software: to forms, public services, medical devices and any process strangers must complete without help. Poka-yoke in particular has spread from car factories to hospitals, where connectors that cannot be attached to the wrong line prevent errors no training could. POSIWID is a useful question for anyone assessing an institution, and Tesler's Law is a useful question for anyone simplifying one: simpler for whom?
Where it is argued over
Some of these rules pull against each other. Postel's Law, be liberal in what you accept, helped the early internet grow, but protocol designers now argue that leniency entrenches other people's errors, which is closer to poka-yoke's strictness. Jugaad is celebrated in management writing as frugal innovation and criticised in India for normalising unsafe workarounds. POSIWID is sometimes misused to attribute intent, as though every outcome of a system were someone's purpose, which Beer did not claim.
Why it is worth knowing
These are rules that fit in one sentence and save months of rework, which is why they circulate. Their deeper lesson is about responsibility: a system that confuses its users, hides its complexity in their laps, or produces results contrary to its stated aims is a design problem, not a user problem. The sayings add the necessary humility, since the gap between a set of parts and a working whole is where most designs fail.
The entries behind this essay: Precept and Figure in Design & Systems. Every subject has its essays on the essays page.