The Rules of Usability

Many collections of user interface guidelines focus on the visual and presentational level of UI design — representative advice of this kind is to avoid using color alone to convey critical information, because about 10% of users have some variety of color blindness. Advice like this has its place, and we'll convey a good deal of it later in the book, but treatments that go at the UI design problem from the outside in often fail to engage deeper issues. The rules we choose to begin this book with are quite deliberately different — structural and logical, working from the inside out and (we believe) grounded in deep properties of the evolved nature of human beings.

These rules of usability are actually rules for programmers, who (almost by definition) are either power users or wizards. They are therefore aimed at programmers looking down the power curve, designing for an audience of domain experts and nontechnical end-users. But as we've already seen, even wizards are in the end-user position much of the time (especially when operating outside the specific realm of their domain knowledge). Therefore, you should consider these rules applicable for designing any program with an interactive interface.

We'll summarize the rules here, noting parallels with the Unix design rules from The Art of Unix Programming. While these rules are implicit in most good treatments of user-interface design, many have never before been written down explicitly. The rest of this book will unfold the implications and consequences of these rules.

Rule of Bliss: Allow your users the luxury of ignorance.

Today's computer users live in an information-saturated society, coping with a frenetic pace of change and living lives that consequently keep their ability to adapt and learn under constant challenge. Under these circumstances, the most precious gift a software designer can give is the luxury of ignorance — interfaces that don't require the consumer to learn another skill, to memorize more details, to shoulder yet another cognitive load.

Simplify, simplify, simplify. Look for features that cost more in interface complexity than they're worth and remove them. When you add features, make them unobtrusive so the user does not have to learn them until he or she is ready (there is a tradeoff here; it's easy to make them too hard to find so that the user never does).

To support the luxury of ignorance, write interfaces that are task-oriented rather than feature-oriented. Task-oriented interfaces walk the user through common sequences of operations, the opposite of the traditional Unix approach that exposes every possible feature switch but forces the user to learn the sequence of operations to accomplish a task. If your program must serve both nontechnical end-users and wizards, then the UI will need to support both task- and feature-oriented interfaces separately. Attempting to combine the two in a single set of controls almost invariably leads to an ugly mess.

Rule of Distractions: Allow your users the luxury of inattention.

One of the effects of an information- and media-saturated world is that people have to budget their attention as well as their learning capacity, and to learn to juggle multiple tasks routinely. Therefore, an equally important gift is the luxury of inattention — interfaces that don't require users to concentrate or change their mental focus from whatever else they are doing. Sometimes this is not a luxury; the task the computer is assisting may demand total attention — but it is nevertheless bad practice to assume that you can get and keep that.

One good test for an interface design is, therefore: can it be worked comfortably while the user is eating a sandwich, or driving a car, or using a cellphone? The real issue here isn't the challenge to physical dexterity but whether the software fits the user's hand and brain well enough to be useful even when the user's attention is divided.

Rule of Flow: Allow your users the luxury of attention.

Sometimes juggling multiple tasks won't do. High performance at writing, programming, and other forms of creative work almost requires human beings to enter a what psychologist Mihaly Csikszentmihalyi has dubbed a flow state — intense but relaxed concentration, with total focus on the task.

Flow states, alas, are very fragile. They take time to establish and are easily broken by distractions. Badly-designed UIs are frequent culprits. Blinking text, graphical animations, popup windows, and intrusive sound effects can be nearly as disruptive as the flow state's deadliest enemy, the ringing telephone.[2]

Well-designed interfaces do not clamor for attention or advertise their own cleverness. Rather, they allow the user to maintain a focus on the task. They support concentration and creativity by getting out of the way.

Rule of Documentation: Documentation is an admission of failure.

The best user interfaces are so transparent and discoverable that they don't require documentation at all. Relying on documentation to explain your program is an admission that you lack the skill and/or imagination to write an interface that good.

And yes, writing programs that don't need documentation is possible. Computer-game designers manage it all the time, even for games that have command repertoires comparable in complexity to a word processor or spreadsheet. The key is crafting interfaces that only require the user to grasp a few operations initially, and are then highly discoverable and lead the user gradually towards mastery. Macintosh programmers have a well-developed tradition of designing UIs in this style.

The Unix experience is actively misleading when it comes to writing for end-users. Traditionally we provide powerful, feature-rich programs for technically adept and highly motivated users who are well up the power curve, and we're very good at that. Those users can benefit from documentation, especially the kind of terse but complete reference found on a good Unix manual page. Novices and nontechnical end-users at the bottom of the power curve are, however, a very different audience and demand different design assumptions.

It is not quite true that end-users never read documentation, but it is true that those who need documentation most are the least motivated and least likely to read it. Also, far too many of the (relatively few) programmers who write documentation fall into using “It's in the documentation!” as an excuse for opaque and poorly-thought-out UI designs.[3] If the Unix community is going to take usability seriously, this kind of laziness can no longer be acceptable.

For both these reasons, you should learn to treat documentation as an admission of failure — sometimes necessary for advanced topics and reference, but never to be relied upon in educating novices and nontechnical end-users.


If the easiest programs to use are those that demand the least new learning from the user, then the easiest programs to use will be those that most effectively connect to the user's pre-existing knowledge.

Therefore, avoid gratuitous novelty and excessive cleverness in interface design. If you're writing a calculator program, ‘+’ should always mean addition! When designing an interface, model it on the interfaces of functionally similar or analogous programs with which your users are likely to be familiar. More generally, the best interface designs match a pre-existing model in the minds of their user populations.

It is especially important to be conservative and imitative about minor features that users have a relatively low incentive to learn. If you must invent a novel way of doing things, the least bad place to do it is right at the center of your design. Suppose, for example, that you are writing a program to generate reports from a database. Writing your own font-selection dialogue would be a bad idea, even if trying a radical new approach to specifying database searches might be a good one.[4]

(This is similar to the Rule of Least Surprise from [TAOUP]. It is also widely known under a slightly different title, as the Principle of Least Astonishment.)

In order to apply this rule, you need to know your audience — to have a model of how your users think and react. If your program has more than one potential audience, you need a mental model for each one.

Modeling your audience in the abstract is surprisingly difficult. To do it you have to step outside your assumptions and forget what you know, and programmers are notoriously bad at this. Fortunately, there is a simple tactic that makes this much easier; put a face on the problem. To serve an audience, make up a fictional character representative of that audience. Tell stories about that character and use those stories to get inside the character's mind (this technique is very like method acting).

Rule of Transparency: Every bit of program state that the user has to reason about should be manifest in the interface.

Hidden state denies users the luxury of inattention, because it means they have to hold things in their heads. An example familiar to many Unix users is older versions of the vi editor, which had no visual indication of whether one was in text-insertion or command mode. It was notoriously easy to forget and insert garbage — or, worse, to perform an unintended edit operation by attempting to insert text.[5]

Mindspace is much more scarce and precious than screen space. Any piece of hidden state your program has is competing for one of those 7±2 slots in short-term memory (see the Rule of Seven below and the extended discussion of this number in the Wetware chapter). In a Unix environment, your program is likely to be one of several among which the user is time-sharing his attention; this makes competition for those slots all the fiercer. But interface design is not a game to be won by claiming those slots — to the contrary, you've done your job best when the user is freed to allocate them himself.

(This is analogous to, but somewhat different from, the Rule of Transparency in [TAOUP].)

Rule of Modelessness: The interface's response to user actions should be consistent and never depend on hidden state.

This rule could be taken as a consequence of the Rule of Transparency, but violations are so common and such an important cause of interface problems that we have chosen to foreground it by making it a separate rule.

An interface with “modes” responds to the same input gesture in different ways depending either on hidden state not visible to the user, or on where you are within the interface. Since the early days of the Macintosh, which eliminated mode dependencies almost completely, it has been widely understood that modes are a bad idea.

The classic bad example in Unix-land is vi(1), a commonly-used editor in which typed characters are either inserted before the cursor or interpreted as editor commands, depending on whether or not you are in insert mode. This is a well-known cause of confusion to novices, and even experts at this editor occasionally trash portions of their documents through having lost track of what mode they are in or inadvertently switched modes. Modern versions of vi(1), partly mitigate this design flaw by providing a visible mode indicator. We know this is not a complete solution, because even expert users can overlook the state of the mode indicator and find themselves in trouble.

We'll see in the Wetware chapter that modes interfere with the process of habituation by which users become expert with interfaces; indeed, they turn habituation into a trap. A computing system ideally designed for human use would have one single set of gestures and commands that are unformly applicable and have consistent meanings across its entire scope.

Rule of Seven: Users can hold at most 7±2 things at once in working storage.

The “Working storage” we're referring to is human short-term memory; the analogy to working storage in a virtual-memory operating system is (as we'll see in the Wetware chapter) both deliberate and fruitful.

This magic threshold of seven items has many applications in UI design. We've already noted one, which is that tracking any hidden state in a program is going to take up one or more of these slots.

It also implies a restriction on how many controls you should have visible at any one time. While users may be able to visually recognize more than seven controls, actually using them will involve refreshing short-term memory with retrieved knowledge about them. If that retrieved knowledge doesn't fit easily in the working store, the cognitive cost of using the interface will go up sharply.

(We'll develop more consequences of the magic number seven in the Wetware chapter.)

Rule of Reversibility: Every operation without an undo is a horror story waiting to happen.

Users make mistakes. An interface that doesn't support backing gracefully out of those mistakes is not a friendly one. Furhermore, absence of an undo inhibits users from exploring and mastering an interface, because that absence makes them wary of doing something irreversibly bad.

Sometimes irreversibility is unavoidable. When you send a mail message there is a commit point past which, having handed a message to the mail infrastructure. you cannot recall it. When you format a hard drive you are physically erasing information that cannot be reconstructed. There are a few applications, such as on-line payment systems, in which irreversibility is a design requirement.

But these are the exceptions, not the rule. Most applications are more like editing text or graphics; the program is a tool with which the user shapes or transforms some kind of database or document. The inexpensive storage and computing power of today's machines allows editor-like applications to keep complete change histories for at least the current session (if not for the entire life of the database/document), and to allow the user to move back and forward in those histories. This is better UI design than having lots of irreversible commit points with confirmation prompts.

Rule of Confirmation: Every confirmation prompt should be a surprise.

On those occasions when you cannot avoid irreversible commit points, it is very bad practice to have confirmation prompts for which the normal answer is “Yes, proceed onwards”. Thus, routine confirmation prompts are a bad idea; when confirmation prompts happen at all, they should always be surprises that make the user stop and think.

The problem with routine confirmation prompts is that they condition users to reflexively click “yes” just to get over that hurdle. There always comes a day when that reflexive “yes” leads to grief. Don't train your users to have bad reflexes, because even if it doesn't cause them problems when using your program, it may well lead them into destructive errors while using someone else's.

Rule of Failure: All failures should be lessons in how not to fail.

An important aspect of discoverability that often gets short shrift is that it should be easy to learn from failure as well as success. A special hell awaits the designers of programs whose response to errors is a message or popup giving a hex code, or one cryptic line that simply says “An error occurred...”.

Documentation can fail this way, too. The most frustrating and useless documentation is the kind that describes in exact detail what normal operation is like (complete with pretty color screenshots of all the screens the user is going to see anyway), but gives no hints about how to recover when an operation does not succeed. And the uttermost depths of bad user experiences are plumbed by programs that fail cryptically, then point the user at documentation that doesn't explain how to cope.

In a well-designed UI, all failures are informative. There are no brick walls; the user always has a place to go next and learn more about the failure and how to recover from it.


One of Unix's oldest and most persistent design rules is that when a program has nothing interesting or surprising to say, it should shut up. Well-behaved programs do their jobs unobtrusively, with a minimum of fuss and bother. Silence is golden.

(This is the Rule of Silence from [TAOUP].)

This rule could be considered a derivative of the Rule of Flow. Many other programming traditions routinely violate it, fatiguing users with a barrage of spurious interrupts that carry little or no real information. Sometimes this happens because the interface designer can't resist showing off his technique. More often, it happens because a lot of activity is good for grabbing the customer's attention at a demo or sales pitch, and the software is designed to be sold rather than used.

When you can't be silent, be clear but unobtrusive. Having part of your program's display quietly change color and display a message is better than a loud, noisy popup that forces the user to take his mind off the task in order to dismiss the popup.

The very height of perversity is reached by programs that interrupt the user's concentration under the pretext of being helpful — in a society that really valued flow states and respected concentration, the inventor of the animated ‘assistant’ in Microsoft Word would have been torn to pieces by an enraged mob. If you remember that allowing the luxury of attention is an important part of your job as a software designer, you won't repeat the Clippy mistake.


A well-designed hardware/software combination is a mind amplifier in precisely the same sense that a lever is a muscle amplifier, The point of computers is to free humans from having to do low-level calculation and information-shuffling — to serve humans. Every time you require a human user to tell a computer things that it already knows or can deduce, you are making a human serve the machine.

Any time you make the user enter magic data that is used solely for bookkeeping within or between computers, you are violating this rule. A prime example is IPV4 dot-quad addresses; the fact that users occasionally have to type these is a clear-cut failure in UI design. Indeed, the fact that users ever have to know about numeric IP addresses at all (as opposed to tagging their network hosts by name or having IP addresses be autonfigured) is a UI design failure of some magnitude. It has not normally been thought of as one, because most of the burden falls on network adminstrators and programmers; but techies are people too, and the amount of skilled time that has been wasted because IP addresses are difficult to remember is undoubtedly immense.

Another excellent reason not to require users to tell the computer anything twice is that it invites inconsistencies. Suppose for example that your program requires the user to enter his/her email address in two different places; this creates the certainty that someday some user is going to typo one of them, or update one and forget the other.

The chance that inconsistencies of this kind will be spotted and fixed before they do damage falls as the number of places the data can be inconsistent rises. Database designers have a rule that there should be a unique SPOT (Single Point of Truth) for each piece of data; good user interfaces have the same property for the same reason.

Rule of Defaults: Choose safe defaults, apply them unobtrusively, and let them be overridden if necessary.

Autodetection is the best thing — the ideal user interface deduces what the user wants to do and does it. But autodetection can become a problem if the computer guesses wrong and there is no way to override the guess. This is one of the most common and damaging UI mistakes, especially among designers who don't have the Unix-tradition bias towards allowing users to configure everything.

Therefore, the best thing is usually for autodetection to supply defaults which the user can override. One of the central design issues for any good UI is how deep to bury the place where overriding any given parameter is possible. If it's too close to the immediately-accessible front of the interface, the user will often be burdened with a detail that is not normally relevant. If it's buried too far in the back (down a deep tree of menus or options), the user may never find it even when he/she needs it.

Rule of Respect: Never mistake keeping things simple for dumbing them down, or vice-versa.

In [TAOUP], a book about designing programs, we emphasized that “Keep It Simple, Stupid!” is the unifying meta-principle behind Unix's design rules. in part because programmers all too frequently overestimate their own capacity to handle code complexity. KISS is the unifying thread in designing good user interfaces, too, if for a slightly different reason: interface designers (especially if they are programmers) tend to overestimate everyone else's capacity to handle interface complexity as well.

With user interfaces as with code, simple designs are actually more intelligent because they avoid the edge cases and failure modes that dog complex designs. But many programmers who understand this about the internals of code are, unfortunately, resistant to the same insight when it comes to UI designs — they will cheerfully pile on controls, each one defensible in itself, until the overall complexity of the interface makes it nigh-impossible for anyone but a programmer already involved with the development to understand.

The only effective counter to this tendency is a ruthless commitment to keeping interfaces simple — simpler than they would need to be for the developer's own use.

Keeping things simple does not equate to dumbing them down. Removing capabilities or obsessively handholding the user in order to “simplify” an interface is a poor substitute for packaging its capabilities in a clean and usable way. But it is equally misguided and lazy to attack simplifications of an interface by claiming that they necessarily dumb it down. The test for a good simplification should always be the same — whether or not it makes the user experience better — and that test should be checked with real users.

Rule of Predictability: Predictability is more important than prettiness.

Remember that predictability — a user's ability to correctly model the operation of the program and assimiliate it to what he or she already knows — is far more important than visual appeal, and that you don't get usability from mere prettiness. Beware of pushing pixels around too much.

One of the most common failure modes in UI design is to create interfaces that are superficially pretty but difficult to use (horrible examples abound, and we will cite some later in this book). Conversely, there are interfaces that are visually cluttered and ugly but highly usable (we'll cite examples of this as well). The latter failure mode is less common, because it is much easier to polish the appearance of a UI than it is to engineer in better usability.

Rule of Reality: The interface isn't finished till the end-user testing is done.

Far too many programmers who would never consider shipping a library without a test suite are somehow willing to ship programs that feature an interactive UI without testing them on real users. This is the blunder in practice that makes violations of all the other rules keep on giving pain — developers don't test, so they either never find out how bad their programs' usability is or find it out after they have too much invested in the interface design to change it easily.

User testing later in the process will be much less an ordeal if you make some effort to be in contact with end-user reality while you're designing. Go out and talk to people who are likely to use the thing. Slap together a quick prototype and get them to complain about it at length. Get a piece of paper and ask them to draw the interface they want with a pencil. You don't have to implement exactly what they say, but it lets you know what they're thinking rather than guessing at it.

Another way to make the test phase more rapid and effective is to apply heuristic evaluation after you have a prototype but before you start looking for test subjects. Later in this book we'll describe a way to do that, which is not a substitute in itself for end-user testing but can go a long way in reducing the amount of it you'll need before converging on a good result.

[2] Or the person from Porlock...

[3] You should then write the documentation anyway, as an exercise in humility, because nobody's perfect. Just don't rely on anyone ever reading it except in very unusual circumstances.

[4] The insight that the best place to innovate is right at the center comes from [Lewis&Rieman].

[5] Newer versions of vi have a visible insert-mode indicator.