# All X is Y but not all Y is X [duplicate]

I am looking for a specific word to describe a concept such that its meaning is linear and one-way. Such is the case for the title of this question. X would be of this concept and Y would not, Y would be the opposite of this concept.

For example, all automobiles are motor vehicles but not all motor vehicles are automobiles.

I cannot seem to find any word to relate to this.

Any advice/guidance is very much appreciated.

-

## marked as duplicate by RegDwigнt♦Apr 30 '14 at 10:23

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

A banana is a type of fruit. – Peter Shor Apr 26 '14 at 22:43
Your question is vague. What do you mean by "linear and one-way"? Why is X, and not Y, of this concept? What concept? – SEL Apr 26 '14 at 23:10
"Opposite" is not a well-defined term for most properties. What is the opposite of the Eiffel Tower? Only linear categories can have opposites. See Ken Hale's Notes on a Walbiri tradition of antonymy – John Lawler Apr 27 '14 at 0:35
All squares are rectangles, but not all rectangles are squares. I realize that's not a single word, but that readily-understood analogy is one I've used several times in situations you're describing. Others like that one, too. By the way, some programmers might call this inheritance. – J.R. Apr 27 '14 at 7:34

## 3 Answers

X is a type, a category, or most generally, a subset of Y.

Conversely, Y is a supercategory or superset of X.

-

In linguistics, the concept is called hyponymy. "Automobile" is a hyponym of "motor vehicle"; conversely, "motor vehicle" is a hypernym of "automobile".

An easy way to remember which one's which is to keep in mind that just like with other words starting with hyper- and hypo-, the former stands for "more, greater", and the latter means "less, smaller". Hypernym is for the bigger superset, while hyponym is for the smaller subcategory.

And of course a hypernym, in turn, can be a hyponym of something else, a bigger superset still. So in your example, "motor vehicle" could be a hyponym of "vehicle", which in turn could be a hyponym of "noun", which in turn could be a hyponym of "word".

So with this, you have a complete set of accurate terms, one for each side in the relationship as well as one for the relationship itself. The terminology is well-established, though the general public is more familiar with some of the other -nyms and -nymies, for other relationships, like synonyms or metonymy. Still, this site for example does have a rather well-populated dedicated tag for questions about hypernyms.

-

Implies works I think. A implies B means that if A is true then B is true but it doesn't necessarily mean B implies A. If both were true you would say A is true if and only if B is true.

So for your example: A is a automobile implies that A is a motor vehicle.

-
You might want to make that X and Y? Good answer, though. :) – medica Apr 27 '14 at 8:06