Downstairs, Upstairs, Headquarters, No Singular Form?

[yiuho]

I was wondering if we say downstairs, Upstairs and Headquarters, can we use singular form? like downstair, upstair, headquarter, as I checked the Longman, Oxford Dictionary, they only have plural form, there's no singular form, hope someone can answer my question. Thanks.

William

[Josef]

Upstairs, downstairs are the only forms.
However, they are singular, not plural:
The upstairs is cleaner than the downstairs.

Headquarters (as a noun) is the only form. It can be treated as plural or singular:
The company's headquarters are in Bristol.
The company's headquarters is in Bristol.

The purist would probably opt for plural.

[yiuho]

Upstairs, downstairs are the only forms.
However, they are singular, not plural:
The upstairs is cleaner than the downstairs.

Headquarters (as a noun) is the only form. It can be treated as plural or singular:
The company's headquarters are in Bristol.
The company's headquarters is in Bristol.

The purist would probably opt for plural.

Hi Josef, thank you very much for your answer. But I'd like to know how come some dictionaries show upstair,downstair,headquarter, can I say these dictionaries incorrect?

[Josef]

The adjective is usually "downstairs" but "downstair" is possible:

a downstairs room
a downstair room

[yiuho]

The adjective is usually "downstairs" but "downstair" is possible:

a downstairs room
a downstair room

Hi Josef, this is very confusing to me as I checked with famous dictionaries, like Longman, Oxford, Cambridge, they never mention downstair, upstair, headquarter without "s", but I checked with some little-known dictionaries, they mention downstair, upstair and headquarter. Long time ago, I watched the English Teaching Video in my country, the well-known English Teacher said downstair, upstair and headquarter must have "s" in anytime. Anyway, thank you for your answer.....

William