depend

The root depend comes from Latin dependēre, built from de- ('down') + pendēre ('to hang'). Its first meaning was wonderfully physical: to hang down from something. A chandelier hangs down from the ceiling; a fruit hangs down from a branch. The branch holds it up — and that is the seed of every modern meaning.

Because a hanging thing relies on what holds it, dependēre slid from 'hang from' to 'rely on.' If you depend on someone, you are figuratively hanging from them, trusting them to bear your weight. That single metaphor — hanging = relying — generated the whole family through prefixes and suffixes.

Make the reliable person an adjective and you get dependable: depend + -able = 'able to be depended on,' someone steady enough that you can safely hang your trust on them. Turn the relationship into a noun and you get dependence: the state of hanging from / relying on another — used for everything from economic dependence to drug dependence (a body that now hangs on a substance to function).

Now add the negative in- ('not'). Independence literally means 'not hanging from anything' — standing on your own, with nothing holding you up but yourself. A colony that wins independence cuts the cord; an independent person, party, or variable answers to no external support. This is the most common member of the family, and its logic is exact: in- cancels the hanging.

Finally, the prefix inter- ('between, mutually') gives interdependent: two or more things that hang from each other, each supporting and relying on the other. Ecosystems, economies, and team members are interdependent — pull one strand and the others sway.

So the family is one image bent five ways: things that hang from something else (depend, dependable, dependence), things that hang from nothing (independence, independent), and things that hang from each other (interdependent). Note the root proper is pend ('hang'); depend is simply its most familiar prefixed form.