headquarter, headquarters, headquartered

The noun headquarters can be either singular or plural. In both cases, it is written with an s at the end; headquarter as a noun is incorrect.

The noun headquarters is generally singular:

• National Defence Headquarters occupies a building complex in downtown Ottawa.
• The headquarters of London Life was originally in London, Ontario.

However, it can be used in the plural if it refers to the offices of several companies or to offices in more than one location:

• The headquarters of many Canadian banks are in Toronto.

Since headquarters is a count noun, it can be modified by any number, including one:

• The command centre comprises six regions and one headquarters.

However, because one headquarters is awkward-sounding, it is best to rephrase the sentence to make it sound more natural:

• The command centre comprises six regions and a national headquarters.
• The command centre comprises six regions and their headquarters.

Note that the verb phrase to be headquartered is used in business and finance:

• Is the Bank of Montreal still headquartered in Montréal?