sign/mark/symbol

[Nefertiti]

We use "X" as the ---for an unknown quantity.

1. sign
2. mark
3. symbol
4. any better suggestions?

Pick the appropriate answer(s). Please explain your answer especially for those don't fit.

Thanks

[Soup]

Would context help?

Ex: We use the symbol "X" to mark (write), or represent an unknown quantity.

[Nefertiti]

Hi Soup.

Is it OK to say the following:

We use the sign "X" to represent an unknown quantity.
We use the mark "X" to represent an unknown quantity.

Why not if the answer is NO.

Thanks for the reply.

[Soup]

All three work, but symbol 'X' is a collocation.

[Nefertiti]

Hi soup.

What's a collocation? Please explain.

Why isn't the mark/sign X a collocation?

Thanks for the prompt reply.

[Soup]

To mark an X or to sign an X are also collocations, but for these the words 'mark' and 'sign' function as verbs. The symbol "X" can also be called 'a mark' or 'a sign'; however, the pattern 'symbol X' is more common, which is why it is called a collocation; i.e., words that often seem to occur naturally together.

For a definition and examples, click here Collocations
For exercises, click here Collocations

[Nefertiti]

Hi soup.

Is it OK to say the following?

1. We mark an "X" to represent an unknown quantity.

2. We sign an "X" to represent an unknown quantity.

3. We mark an "X" for an unknown quantity.

4. We sign an "X" for an unknown quantity.


Thanks

[Soup]

Yes. They're all OK, but 'use the symbol X' is more common/commoner.