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Dependency is directional. Starting a car is dependent on petrol. But petrol is not dependent on starting a car.

What word describes the relationship from petrol to the car?

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3 Answers 3

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requisite (n., adj.)

A necessary or indispensable thing; something needed for a particular purpose.

All the requisites for living are there if you take the time to look. OED

ADJECTIVE
You can use requisite to indicate that something is necessary for a particular purpose.
[formal]
She filled in the requisite paperwork.
Synonyms: necessary, needed, required, called for

COUNTABLE NOUN
A requisite is something that is necessary for a particular purpose

[formal]
An understanding of accounting techniques is a requisite for the work of the analysts. Collins

If you really need something opposite to dependency, requisiteness does exist:

The fact of being requisite; necessity, need.

Focusing on the preexisting social facticity of possessive individualism..and its requisiteness to the functioning of market society. OED


... and with arguing that certain sorts of relationships are 'requisite' in terms of the tasks which have to be achieved. There is no systematic evidence in terms of which such claims as to the requisiteness of the advocated patterns can be tested... R. K. Brown; Understanding Industrial Organizations

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You hinted at one yourself with "directional." The phrase "unidirectional relationship" seems to be used mainly in the fields of computing logic (and apparently psychology?), but has exactly the definition you give. For a more informal equivalent, you could just say "one-way relationship."

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  • This seems to be a comment on the wording of the question, rather than an answer to it.
    – jsw29
    Aug 27, 2021 at 16:05
  • Really? Sorry, perhaps I was unclear. "Unidirectional relationship" or "one-way relationship" express what the OP is looking for (to a T, it seems to me). Aug 27, 2021 at 16:22
  • The OP was looking for a word that can fill the blank in 'If starting a car is dependent on petrol, then petrol is ____ for starting a car'.
    – jsw29
    Aug 28, 2021 at 15:49
  • @jsw29 Sorry, I took it differently. "What word describes the relationship." I take it they're looking for an adjective that modifies the relationship, not the petrol. Aug 28, 2021 at 16:54
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  1. Mutual dependence. Mutually dependent.
  2. Interdependence. Interdependent.

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