This is the strongest argument against everything else on this site, so it gets its own page rather than a footnote. Hierarchy buys interpretability and progressive disclosure. What it sells, in exchange, is error detection — and that is not a small thing to sell.
What what3words does, and why
what3words is deliberately non-hierarchical and non-topological. Every square is the same size, so there is nothing to interpret; and the three words for a square have no relationship to the words for its neighbours. The algorithm actively scatters similar-sounding combinations across the globe, so that a one-word slip lands you on another continent.
What hierarchy does instead
In a nested scheme a wrong word necessarily lands you nearby, because the words above it still constrain where you are. That makes the mistake quiet: silently plausible rather than obviously wrong.
| Word got wrong | Still constrained to | Worst-case displacement |
|---|---|---|
| Word 1 — Region | nothing; anywhere in the metro | 84.9 km |
| Word 2 — Locality | the level-1 cell | 7.1 km |
| Word 3 — Spot | the level-2 cell | 354 m |
Worse, the semantic layout concentrates the risk. Because words that mean similar things are placed next to each other, the substitution a human is most likely to make is also the one that moves them least. For the address trough.carton.duckling, the six nearest confusions in meaning are:
| Mistaken address | Word changed | Displacement |
|---|---|---|
trough.carton.swan | word 3 → swan | 28 m |
trough.carton.quail | word 3 → quail | 45 m |
trough.carton.pigeon | word 3 → pigeon | 52 m |
trough.carton.duckweed | word 3 → duckweed | 129 m |
trough.cherry.duckling | word 2 → cherry | 354 m |
trough.celadon.duckling | word 2 → celadon | 1.4 km |
What the design does about it
The trade-off cannot be eliminated — it is structural. Three things reduce it.
- The vocabulary contract. No two of the 936 words in the system are within one Levenshtein edit of each other. A single mistyped character therefore cannot be another valid word: it produces a non-word, which is rejected at parse time rather than resolved to a plausible wrong doorway. This is verified against the shipped lexicon by an exhaustive test that mutates every character of every word to every other letter — over 100,000 mutations, none of which hit a valid word.
- Disjoint vocabularies per level. A word in the wrong position is caught immediately, including the common case of a transposition. A flat scheme with one global vocabulary cannot do this at all — any word is legal in any slot.
- Surfacing the risk in the product. The Risk tab on every address lists exactly these confusions with their distances. Making the failure mode visible is not a fix, but hiding it would be worse.
When this trade is worth making
- Worth it where a near-miss is recoverable and interpretability compounds: last-mile delivery in an unaddressed neighbourhood, field survey, utility inspection, informal-settlement mapping, or anywhere a human will look up and see they are on the wrong side of the street.
- Not worth it for emergency dispatch, offshore rescue, or any single-shot channel where nobody will notice a 200 m error until it matters.
- Uniquely valuable where progressive disclosure is the point: a courier can be given
cavernto reach the district and only receive the full address on arrival. A flat scheme has no way to express “roughly here”.