Depth Follows the Same Pattern as Length and Width
Depth is the distance from top to bottom. English builds it from deep with a th ending, the same way it builds length and strength.
Depth is the distance from the top of something to its bottom, or from front to back. It belongs to a small closed set of English nouns built from adjectives with a *-th* ending and a vowel change: long gives length, strong gives strength, broad gives breadth, wide gives width, deep gives depth. The pattern stopped being productive centuries ago, which is why no one forms new words this way. *Out of your depth* comes from swimming where you cannot stand.
- Vertical distance. *A depth of ten metres.*
- Intensity. *Depth of feeling.*
- Thoroughness. *A study in depth.*
- Out of your depth — beyond your ability. *Out of his depth.*
- Deep — the adjective. *A deep well.*
Word forms
| Form | Word | Example |
|---|---|---|
| Noun | depth | Great depth. |
| Plural | depths | The ocean depths. |
| Adjective | deep | A deep lake. |
| Verb | deepen | The crisis deepened. |
| Adverb | deeply | Deeply moved. |
- The lake reaches a depth of forty metres.Measurement.
- He was out of his depth in the role.Idiom.
- We looked at it in depth.Thoroughness.
The -th nouns
- Deep → depth
- Long → length
- Strong → strength
- Broad → breadth
- Wide → width, the newest of the set and still slightly irregular
Middle English, formed from *deep* on the pattern of *length*, from Old English *deop*. The noun form is not attested in Old English itself.
Why is it depth and not deepness?
Depth follows an old pattern shared with length and strength. Deepness exists but sounds unusual and is rarely used.
What does out of your depth mean?
Beyond your ability to cope, from swimming in water too deep to stand in.