apt

The root apt comes from Latin aptus, meaning "fitted, suitable, joined together." Aptus was originally the past participle of the verb apere, "to fasten, to attach." So at its core, apt is about two things fitting together — like a key sliding into a lock, or a piece of clothing tailored exactly to your body. Whenever something is fitted to its purpose, the root apt is at work.

In English, the bare adjective apt keeps this original sense: an apt remark is one that fits the situation perfectly. From this idea of fitness, the family spreads in two directions.

First, the direction of natural fitness — being well-suited from birth. aptitude is your built-in fitness for something: a natural ability, a talent you were born "fitted" for. An aptitude test measures what you're naturally suited to do.

Second, the direction of making things fit — and this is where the prefixes come in:

- ad- (to, toward) + aptus → adapt: to make something fit a new situation. When you adapt, you reshape yourself or a thing so it fits changed conditions. A novel adapted for film is refitted for the screen; an animal adapts to its environment by becoming better-fitted to survive.
- ad- (to) + aptus, through a different path, also gave adept — literally "having attained" mastery, someone so well-fitted to a craft they handle it effortlessly. (It was once an alchemist's term: an adeptus was one who had "attained" the secret art.)
- in- (not) + aptus → inept: not fitted, not suited — clumsy, bungling. Notice the spelling shift: aptus becomes -eptus inside the word, which is why we say inept and adept with an e, but apt and adapt with an a. This is the variant ept.

So the family lays out neatly along the question "does it fit?":
- apt — it fits (suitable)
- aptitude — born to fit (natural talent)
- adapt — make it fit (adjust)
- adept — fits the craft perfectly (highly skilled)
- inept — doesn't fit at all (clumsy)

One genuinely surprising member: attitude. It looks unrelated, but it traveled a winding road. Latin aptitūdō ("fitness, aptness") passed into Italian as attitudine, meaning the "fitness" or disposition of the body — a pose, a posture suited to a moment. Painters borrowed it for the attitude of a figure in a painting. From the body's physical posture, English extended it to a mental posture: your attitude is the stance your mind takes toward something. So attitude and aptitude are actually the same Latin word, split apart by centuries and two languages.