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What's the difference between describing a number or place as cardinal versus ordinal? They both seem to have to do with the order of things. For example, when saying "He is the first in his class", is first ordinal or cardinal? What are the usage differences here?

  • In standard everyday English, I find these hard to distinguish, 'three' vs 'third'...big deal, they're pretty much the same thing. Yes, cardinal is for counting, and ordinal is for ordinal, but as you count, you've gotten to the ordinal. Same thing. However, there is a very technical meaning to these terms in mathematics, and it might be more satisfying to re-ask this question in [math.stackexchange.org](math.stackexchange.org). – Mitch Jun 3 '11 at 3:09
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    @Mitch: Well, there can be a number of objects that are in no particular order (see the last paragraph of my edited answer). Any number of objects can be designated by a cardinal number; but only if they are in some kind of order can ordinals be used. – Cerberus_Reinstate_Monica Jun 4 '11 at 3:39
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    This question can be answered by a dictionary lookup: Oxford Dictionaries entries for cardinal number (examples one, two, three) and ordinal number (examples first, second, third). – herisson May 25 '16 at 17:12
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Cardinalis means "that upon which a door hinges, pivotal" in Latin, from cardo, "hinge, pivot". From this it acquired its secondary meaning, "important, principal", which it still has in English (e.g. a cardinal sin). Its third meaning is derived from this: a cardinal number is a "principal" number, i.e. one that simply says how many objects there are.

That'll be one pound.

Two billion people might die.

Ordinalis means "in order of succession, of an order". It comes from ordo, "order, rank". An ordinal number is an adjective that denotes what place an object has in a certain order. The names of the ordinal numbers are usually derived from the cardinal numbers by adding -th.

That is my second victory.

This is the tenth time she's dumped me.

While a cardinal number refers to several objects ("three apples"), an ordinal number refers to only one of those ("the third apple"). An ordinal number is hence dependent on the notion of a cardinal number: there can't be a third apple unless there are at least three apples. By contrast, there can be three apples without one being the third, if they are just not arranged in any particular order.

The Romans used these terms the same way. They also had distributive numbers, which indicated "every third apple", or apples "in triads", "three each".

  • And Latinate (e.g., primary, secondary), as a bonus? – Dave Jarvis Oct 7 '16 at 4:57
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Cardinal has to do with cardinality or the magnitude or quantity of things.

Ordinal has to do with ordinality or the ordering or ranking of things.

Thus, first is an ordinal number. Its cardinal equivalent is one.

  • @RegDwight: Thanks for the edit. That was embarrassing. – Dancrumb Jan 17 '12 at 22:14
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Quite easy. The noun is placed before adjective in cardinal. Whereas in ordinal the noun is put after the adjective e.g. 'class one' and '1st class' so, class is noun while 'one' was cardinal and '1st' was ordinal.

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    That's not true in most varieties of English that I know of. Cardinal or ordinal, they are both adjectives and put in the common adjective position of before the noun. – Mitch Jan 11 '12 at 19:25

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