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Tesler's Law

STR·10 · 10 of 11

Every task has an irreducible complexity; the only question is who absorbs it — the system or the person.

traditional Long-standing practice, with no single citable origin. Why: Tesler’s law of conservation of complexity is an industry formulation, widely attributed and not published as a study.

The same task, built twice

Both panes below are real interfaces. They respond, they can be got wrong, and each is timed from your first move in it. The times are not a study — two panes, one reader, no controls. They are here to make a difference you can feel into a number you can see, and nothing is recorded or sent anywhere.

Your task

Enter the date 3 March 2027 and the phone number 07700 900123.

Complexity pushed out—

Try “3/3/27”. Try “07700900123”. The rules exist and are not written down, so the only way to find them is to fail. Every rejection here is work the system decided not to do.

Complexity absorbed—

3/3/27, 03-03-2027, 3 Mar 2027 and 2027-03-03 all work.

Spaces, dashes and +44 are all fine.

Same two facts, same two fields, and the parsing happens on this side of the screen. The formats it accepts are listed before you type, not after you get it wrong.

What to watch The complexity did not go anywhere — a date still has to become a date. Tesler's claim is that someone pays for it, and the only question a designer actually gets to answer is who.

The principle

OriginLarry Tesler, at Xerox PARC and after — the conservation of complexity.
EvidenceEvery "simple" interface conceals machinery that ate the complexity on the user's behalf: one-tap payment stands on a mountain of absorbed difficulty. The converse is the form that makes you do the parsing the system declined to.
Limits & misuseNot all complexity is conserved — some is invented, and should be deleted, not moved. And absorbing complexity has a price the principle doesn't mention: the system that hides everything also hides what went wrong (see: every inscrutable error the basement collects).